Fix computed value testing for offset-rotate keywords

The default keyword testing does not handle the special computed values of
offset-rotate correctly. For example, "reverse" computes to "auto 180deg",
while "auto" may be exposed as either "auto" or "auto 0deg" depending on the
engine.

Add custom computed value checks for these keyword cases so the tests match the
property-specific computed value behavior.

Differential Revision: https://phabricator.services.mozilla.com/D298938

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2037446
gecko-commit: f3c574159a013e064289c870860bfb67ad0178ba
gecko-commit-git: 58dc4f5be7b75198c23a35d5423c5df1bb5567cf
gecko-reviewers: emilio, firefox-style-system-reviewers

Author: janvarga

css/css-typed-om/the-stylepropertymap/properties/offset-rotate.html

@@ -13,8 +13,20 @@
 'use strict';
 
 runPropertyTests('offset-rotate', [
-  { syntax: 'auto' },
-  { syntax: 'reverse' },
+  {
+    syntax: 'auto',
+    computed: (input, result) => {
+      try {
+        assert_is_unsupported(result);
+      } catch(e) {
+        assert_style_value_equals(result, input);
+      }
+    }
+  },
+  {
+    syntax: 'reverse',
+    computed: (_, result) => assert_is_unsupported(result)
+  },
   {
     syntax: '<angle>',
     specified: (input, result, alternateExpected) => {