No bug - update WPT expectations for servo/servo#20224. r=me

This is a test change that hasn't been synced yet:

  https://github.com/w3c/web-platform-tests/pull/9862

Once it is it'll start passing.

MozReview-Commit-ID: H0RZlep7oX3

testing/web-platform/meta/css/cssom/serialize-values.html.ini

@@ -1,4 +1,8 @@
 [serialize-values.html]
+  [content: counter(par-num)]
+    expected:
+      if stylo: FAIL
+
   [orphans: 101]
     expected: FAIL