Fix test failures that occur when nokogiri is using system libxml:

  https://github.com/rgrove/sanitize/issues/198

Taken from upstream:
https://github.com/rgrove/sanitize/commit/21da9b62baf9ea659811d92e6b574130aee57eba

diff --git a/test/test_malicious_html.rb b/test/test_malicious_html.rb
index 2c23074..0756de0 100644
--- a/test/test_malicious_html.rb
+++ b/test/test_malicious_html.rb
@@ -135,6 +135,8 @@
   # The relevant libxml2 code is here:
   # <https://github.com/GNOME/libxml2/commit/960f0e275616cadc29671a218d7fb9b69eb35588>
   describe 'unsafe libxml2 server-side includes in attributes' do
+    using_unpatched_libxml2 = Nokogiri::VersionInfo.instance.libxml2_using_system?
+
     tag_configs = [
       {
         tag_name: 'a',
@@ -166,6 +168,8 @@
         input = %[<#{tag_name} #{attr_name}='examp<!--" onmouseover=alert(1)>-->le.com'>foo</#{tag_name}>]
 
         it 'should escape unsafe characters in attributes' do
+          skip "behavior should only exist in nokogiri's patched libxml" if using_unpatched_libxml2
+
           # This uses Nokogumbo's HTML-compliant serializer rather than
           # libxml2's.
           @s.fragment(input).
@@ -191,6 +195,8 @@
         input = %[<#{tag_name} #{attr_name}='examp<!--" onmouseover=alert(1)>-->le.com'>foo</#{tag_name}>]
 
         it 'should not escape characters unnecessarily' do
+          skip "behavior should only exist in nokogiri's patched libxml" if using_unpatched_libxml2
+
           # This uses Nokogumbo's HTML-compliant serializer rather than
           # libxml2's.
           @s.fragment(input).
input type='hidden' name='id' value='03009f2a2bb5c94e94735d2ff9314502412066a4'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=03009f2a2bb5c94e94735d2ff9314502412066a4'>root</a>/<a href='/guix/log/po?id=03009f2a2bb5c94e94735d2ff9314502412066a4'>po</a>/<a href='/guix/log/po/packages?id=03009f2a2bb5c94e94735d2ff9314502412066a4'>packages</a>/<a href='/guix/log/po/packages/zh_CN.po?id=03009f2a2bb5c94e94735d2ff9314502412066a4'>zh_CN.po</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/po/packages/zh_CN.po?id=03009f2a2bb5c94e94735d2ff9314502412066a4&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-04-02 18:42:36 +0200'>2022-04-02</span></td><td><a href='/guix/commit/po/packages/zh_CN.po?id=8a208df17fe81fbf34f5a86cc52bb0b8e33c7b96'>nls: Update translations.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/packages/fi.po: New file.
* po/packages/LINGUAS: Add it.
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2022-03-01 22:17:18 +0100'>2022-03-01</span></td><td><a href='/guix/commit/po/packages/zh_CN.po?id=8c92d85e06e65c5cdb5f6c75faf8aa33e5a68513'>nls: Update translations.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/doc/guix-cookbook.uk.po: New file.
* po/doc/local.mk: Add uk cookbook.
* doc/local.mk: Add uk cookbook.
* po/guix/fa.po: New file.
* po/guix/uk.po: New file.
* po/guix/LINGUAS: Add fa and uk.
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2022-02-04 18:29:00 +0100'>2022-02-04</span></td><td><a href='/guix/commit/po/packages/zh_CN.po?id=8391a99d089322a39cbacb1e6dc2979d8b2ef7c9'>nls: Update translations.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/doc/guix-cookbook.pt_BR.po: New file.
* po/doc/local.mk: Add it.
* doc/local.mk: Add pt_BR cookbook.
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2022-01-09 11:35:59 +0100'>2022-01-09</span></td><td><a href='/guix/commit/po/packages/zh_CN.po?id=934b0cdfee2078bad42429427e12d6ee2ddb7fec'>nls: Update translations.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/doc/guix-cookbook.fi.po: New file.
* po/doc/guix-manual.fi.po: New file.
* po/doc/local.mk: Add them.
* doc/local.mk: Add them.
* po/guix/fi.po: New file.
* po/guix/LINGUAS: Add it.
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2021-12-04 19:40:52 +0100'>2021-12-04</span></td><td><a href='/guix/commit/po/packages/zh_CN.po?id=7fcb20a8fa242ab53b9bc97ef0b0367c6a399bd4'>nls: Update translations.</a></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2021-11-06 14:06:36 +0100'>2021-11-06</span></td><td><a href='/guix/commit/po/packages/zh_CN.po?id=85430a9ea6f32aaae3e34741e292a01a2de62ee0'>nls: Update translations.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/doc/guix-cookbook.es.po: New file.
* po/doc/local.mk: Add 'es' cookbook.
* doc/local.mk: Add 'es' cookbook.
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2021-10-17 22:10:25 +0200'>2021-10-17</span></td><td><a href='/guix/commit/po/packages/zh_CN.po?id=15c91189cb61c579f4289047c79530cefe75215f'>nls: Update translations.</a></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2021-09-02 04:06:52 +0200'>2021-09-02</span></td><td><a href='/guix/commit/po/packages/zh_CN.po?id=1ff2092a39dc5c7c7cd9b4a0ba4b59f92e738efc'>nls: Update translations.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/packages/it.po: New file.
* po/packages/LINGUAS: Add `it'.
* po/*/*.po: Update translations.
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2021-08-04 14:49:53 +0200'>2021-08-04</span></td><td><a href='/guix/commit/po/packages/zh_CN.po?id=6c10c96fedf707d7be059aeb79bd5c8e2d2fa210'>nls: Update translations.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/doc/guix-cookbook.ru.po: New file.
* po/doc/guix-cookbook.sk.po: New file.
* po/*/*.po: Update translations.
* doc/local.mk (COOKBOOK_LANGUAGES): Add ru and sk.
(info_TEXINFOS): Add Russian and Slovak cookbooks.
* po/doc/local.mk (DOC_COOKBOOK_PO_FILES): Add Russian and Slovak po
files.
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2021-05-10 22:34:43 -0400'>2021-05-10</span></td><td><a href='/guix/commit/po/packages/zh_CN.po?id=21fcced608c93e1ac836cdd9ab8c5d200c667e75'>nls: Update translations.</a></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2021-04-18 13:18:02 +0200'>2021-04-18</span></td><td><a href='/guix/commit/po/packages/zh_CN.po?id=5d1f110b454b9de30a8dde8d51a213bb956d54f3'>nls: Update 'zh_CN' translations.</a></td><td>Julien Lepiller</td></tr>