[mathml] Fix mo-form.html

The codepoint U+266D was used in this test and part of the MathML3
operator dictionary, however it is not part anymore of the MathML
Core operator dictionary. Therefore pick a postfix operator that
is part of the MathML Core operator dictionary.

Bug: 6606
Change-Id: I253909a13a3d1ce34ca597342a706c05d35d3d34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466243Reviewed-by: Frédéric Wang <fwang@igalia.com>
Commit-Queue: Rob Buis <rbuis@igalia.com>
Cr-Commit-Position: refs/heads/master@{#816531}

third_party/blink/web_tests/TestExpectations

@@ -1229,7 +1229,6 @@ crbug.com/6606 external/wpt/mathml/presentation-markup/operators/embellished-ope
 crbug.com/6606 external/wpt/mathml/presentation-markup/operators/embellished-operator-002.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/operators/mo-axis-height-1.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/operators/mo-form-dynamic.html [ Failure ]
-crbug.com/6606 external/wpt/mathml/presentation-markup/operators/mo-form.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/scripts/cramped-001.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/spaces/space-like-001.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/spaces/space-like-002.html [ Failure ]

third_party/blink/web_tests/external/wpt/mathml/presentation-markup/operators/mo-form-ref.html

@@ -17,9 +17,9 @@
     _<math><mo lspace="0.27777em" rspace="0.27777em">⋉</mo></math>_
 
     <!-- This checks the effect of @form on a postfix operator. -->
-    _<math><mo lspace="0.27777em" rspace="0.27777em">♭</mo></math>_
-    _<math><mo lspace="0.27777em" rspace="0.27777em">♭</mo></math>_
-    _<math><mo lspace="0em" rspace="0.11111em">♭</mo></math>_
+    _<math><mo lspace="0.27777em" rspace="0.27777em">”</mo></math>_
+    _<math><mo lspace="0.27777em" rspace="0.27777em">”</mo></math>_
+    _<math><mo lspace="0em" rspace="0em">”</mo></math>_
 
   </body>
 </html>

third_party/blink/web_tests/external/wpt/mathml/presentation-markup/operators/mo-form.html

@@ -20,9 +20,9 @@
     _<math><mo form="postfix">⋉</mo></math>_
 
     <!-- This checks the effect of @form on a postfix operator. -->
-    _<math><mo form="infix">♭</mo></math>_
-    _<math><mo form="prefix">♭</mo></math>_
-    _<math><mo form="postfix">♭</mo></math>_
+    _<math><mo form="infix">”</mo></math>_
+    _<math><mo form="prefix">”</mo></math>_
+    _<math><mo form="postfix">”</mo></math>_
 
     <script src="/mathml/support/feature-detection.js"></script>
     <script>MathMLFeatureDetection.ensure_for_match_reftest("has_operator_spacing");</script>