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).
s='form'><form class='right' method='get' action='/guix/log/gnu/installer/newt/substitutes.scm'>
<input type='hidden' name='id' value='2a9739b114bae2a55f5cdd44006e1fff8c03443a'/><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=2a9739b114bae2a55f5cdd44006e1fff8c03443a'>root</a>/<a href='/guix/log/gnu?id=2a9739b114bae2a55f5cdd44006e1fff8c03443a'>gnu</a>/<a href='/guix/log/gnu/installer?id=2a9739b114bae2a55f5cdd44006e1fff8c03443a'>installer</a>/<a href='/guix/log/gnu/installer/newt?id=2a9739b114bae2a55f5cdd44006e1fff8c03443a'>newt</a>/<a href='/guix/log/gnu/installer/newt/substitutes.scm?id=2a9739b114bae2a55f5cdd44006e1fff8c03443a'>substitutes.scm</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/gnu/installer/newt/substitutes.scm?id=2a9739b114bae2a55f5cdd44006e1fff8c03443a&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-12-11 21:37:07 +0100'>2020-12-11</span></td><td><a href='/guix/commit/gnu/installer/newt/substitutes.scm?id=8361817bf693742757b096468198626f297bb09e'>install: Discover local substitute servers.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/installer/substitutes.scm: New file.
* gnu/installer/newt/substitutes.scm: New file.
* gnu/local.mk (INSTALLER_MODULES): Add them.
* po/guix/POTFILES.in: Add gnu/installer/newt/substitutes.scm.
* gnu/installer/proxy.scm (with-silent-shepherd): Move to ...
* gnu/installer/utils.scm: ... here.
* gnu/installer/record.scm (&lt;installer&gt;)[substitutes-page]: New field.
* gnu/installer/newt.scm (substitutes-page): New procedure,
(newt-installer): register it.
* gnu/installer.scm (installer-steps): Add "substitutes-page" step.
* gnu/system/install.scm (%installation-services): Add avahi-service-type and
enable substitute server discover in guix-service-type.
[&lt;name-service-switch&gt;]: Set it to %mdns-host-lookup-nss.
</span></span></td><td>Mathieu Othacehe</td></tr>