Auto merge of #8601 - mfeckie:feature/manifest-diff-ci-check, r=jdm

Check if MANIFEST.json changes as per #8587

Adds a script which checks if either of the MANIFEST.json files change in `test/wpt` in response to #8587

- Addresses currently incorrect MANIFEST.json
- Adds checking script

PR for saltfs to add to CI checks to follow (https://github.com/servo/saltfs/pull/163)

https://reviewable.io/reviews/servo/servo/8601

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tests/wpt/metadata/MANIFEST.json

@@ -35511,4 +35511,4 @@
   "rev": "5a754b40cd49c0404863c431b58cc311dc5d167c",
   "url_base": "/",
   "version": 2
-}
+}

tests/wpt/mozilla/meta/MANIFEST.json

@@ -9881,7 +9881,7 @@
           "path": "css/viewport_ignore_desktop.html",
           "references": [
             [
-              "/_mozilla/css/viewport_ignore_desktop.html",
+              "/_mozilla/css/viewport_ignore_desktop_ref.html",
               "=="
             ]
           ],