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).
ght' method='get' action='/guix/log/gnu/packages/accessibility.scm'>
<input type='hidden' name='id' value='f678c5f932a0dec3a22abc6a3feb608be59225e6'/><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=f678c5f932a0dec3a22abc6a3feb608be59225e6'>root</a>/<a href='/guix/log/gnu?id=f678c5f932a0dec3a22abc6a3feb608be59225e6'>gnu</a>/<a href='/guix/log/gnu/packages?id=f678c5f932a0dec3a22abc6a3feb608be59225e6'>packages</a>/<a href='/guix/log/gnu/packages/accessibility.scm?id=f678c5f932a0dec3a22abc6a3feb608be59225e6'>accessibility.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/packages/accessibility.scm?id=f678c5f932a0dec3a22abc6a3feb608be59225e6&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2024-05-23 20:39:13 +0300'>2024-05-23</span></td><td><a href='/guix/commit/gnu/packages/accessibility.scm?id=4ab8657b23ed951c6a95d18b4574e39f3aceca32'>gnu: libbraille: Fix build on some architectures.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2024-05-23 20:39:13 +0300'>2024-05-23</span></td><td><a href='/guix/commit/gnu/packages/accessibility.scm?id=36942212908164381231f96a76fcbbf86ac2856a'>gnu: brltty: Build with java only on some architectures.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2024-05-06 23:16:55 +0300'>2024-05-06</span></td><td><a href='/guix/commit/gnu/packages/accessibility.scm?id=3053c40995675c4b7a0c92c001e98de432326766'>gnu: mouseloupe: Update the package style.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2024-05-06 23:16:54 +0300'>2024-05-06</span></td><td><a href='/guix/commit/gnu/packages/accessibility.scm?id=2d5b779b451dc1f6ca244d7760287c9d5f6f2f7e'>gnu: mouseloupe: Don't hardcode building for i686.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2024-02-10 11:04:35 +0000'>2024-02-10</span></td><td><a href='/guix/commit/gnu/packages/accessibility.scm?id=ce302c99bdc7b2df69577faf2abecf72f0ec3234'>gnu: brltty: Fix python package install.</a><span class='msg-avail'>...</span></td><td>Dariqq</td></tr>
<tr><td><span title='2023-08-15 05:45:46 +0300'>2023-08-15</span></td><td><a href='/guix/commit/gnu/packages/accessibility.scm?id=561c8851162441d2d997804cf4afb69ced2edb07'>gnu: florence: Build with librsvg-for-system.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>