Add support for faster reversing of interrupted transitions

This is described in the spec and allows interrupted transitions to
reverse in a more natural way. Unfortunately, most of the tests that
exercise this behavior use the WebAnimations API. This change adds a
test using our custom clock control API.

tests/wpt/mozilla/meta/MANIFEST.json

@@ -12870,6 +12870,13 @@
        {}
       ]
      ],
+     "faster-reversing-of-transitions.html": [
+      "8471a18f962283afd8d6a81c8ab868e5c2eedd7d",
+      [
+       null,
+       {}
+      ]
+     ],
      "mixed-units.html": [
       "bb029a9fa80650c39e3f9524748e2b8893a476e1",
       [