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).
gnu/build/bootloader.scm'>
<input type='hidden' name='id' value='f7947f89916e4d6b764c128f2f26623eb5b6734e'/><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=f7947f89916e4d6b764c128f2f26623eb5b6734e'>root</a>/<a href='/guix/log/gnu?id=f7947f89916e4d6b764c128f2f26623eb5b6734e'>gnu</a>/<a href='/guix/log/gnu/build?id=f7947f89916e4d6b764c128f2f26623eb5b6734e'>build</a>/<a href='/guix/log/gnu/build/bootloader.scm?id=f7947f89916e4d6b764c128f2f26623eb5b6734e'>bootloader.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/build/bootloader.scm?id=f7947f89916e4d6b764c128f2f26623eb5b6734e&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-11-03 07:59:59 +0100'>2020-11-03</span></td><td><a href='/guix/commit/gnu/build/bootloader.scm?id=a38d861e571c952f78ca588b50bdcc4adcb0c88c'>system: reconfigure: Use the disk-installer if provided.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes: &lt;https://issues.guix.gnu.org/44101&gt;.

* gnu/build/bootloader.scm (write-file-on-device): Pass 'no-fail flag instead
of 'no-create. Use a latin-1 transcoder.
* guix/scripts/system/reconfigure.scm (install-bootloader-program): Add a
"disk-installer" argument and use it as a fallback.
(install-bootloader): Adapt accordingly.
* gnu/tests/reconfigure.scm (run-install-bootloader-test): Ditto.
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-10-05 11:10:35 +0200'>2020-10-05</span></td><td><a href='/guix/commit/gnu/build/bootloader.scm?id=b97b423e3f61c80d5877dadc95b3f316cd61788f'>bootloader: Fix u-boot installation.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a follow-up of f19cf27c2b9ff92e2c0fd931ef7fde39c376adaa. The
bootloader installation must be done on the final disk-image, hence using
"disk-image-installer" instead of "installer" callback.

* gnu/bootloader/u-boot.scm: Turn all installer callbacks into
disk-image-installer callbacks.
* gnu/build/bootloader.scm (write-file-on-device): Open the output file with
'no-truncate and 'no-create options.
* gnu/system/image.scm (with-imported-modules*): Add (gnu build bootloader)
module.
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-05-05 16:08:33 +0200'>2020-05-05</span></td><td><a href='/guix/commit/gnu/build/bootloader.scm?id=f8fd1157174fd523d36dcfa756c965a54c30d5ae'>build: bootloader: Add install-efi procedure.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/build/bootloader.scm (install-efi): New procedure copied from (gnu build vm).
(install-efi-loader): New exported procedure, wrapping install-efi.
* gnu/build/vm.scm (initialize-hard-disk): Adapt to use install-efi-loader.
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2019-06-17 16:13:36 +0200'>2019-06-17</span></td><td><a href='/guix/commit/gnu/build/bootloader.scm?id=45d46223f92b0933aaf9b1392a21d09eaa1e2881'>utils: Add 'invoke/quiet'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/build/bootloader.scm (G_): Remove.
(open-pipe-with-stderr, invoke/quiet): Move to...
* guix/build/utils.scm: ... here.  Use 'let-values' instead of
'define-values' because Guile 2.0 (the bootstrap Guile) doesn't know
about 'define-values'.
* po/guix/POTFILES.in: Remove gnu/build/bootloader.scm, and add
guix/build/utils.scm.
* tests/build-utils.scm: Remove import of (gnu build bootloader).
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-03-16 18:15:13 +0100'>2019-03-16</span></td><td><a href='/guix/commit/gnu/build/bootloader.scm?id=f0cc5e7e1e4c03af29c5d4855dc5962502c49147'>booloader: Add 'invoke/quiet'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/build/bootloader.scm (G_): New macro.
(open-pipe-with-stderr, invoke/quiet): New procedures.
* tests/build-utils.scm ("invoke/quiet, success")
("invoke/quiet, failure")
("invoke/quiet, failure, message on stderr"): New tests.
* po/guix/POTFILES.in: Add bootloader.scm.
</span></span></td><td>Ludovic Courtès</td></tr>