servo: Merge #8601 - Check if MANIFEST.json changes as per #8587 (from mfeckie:feature/manifest-diff-ci-check); r=jdm

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

Source-Repo: https://github.com/servo/servo
Source-Revision: f2fe1171d2a4f4ae37cfa0e0e345662f68e41c5d

servo/.travis.yml

@@ -11,6 +11,7 @@ matrix:
       script:
          - ./mach build -d --verbose
          - bash etc/ci/lockfile_changed.sh
+         - bash etc/ci/manifest_changed.sh          
       cache:
         directories:
           - .cargo

servo/etc/ci/manifest_changed.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+./mach test-wpt --manifest-update SKIP_TESTS > /dev/null
+./mach test-css --manifest-update SKIP_TESTS > /dev/null
+diff=$(git diff -- tests/*/MANIFEST.json)
+echo "$diff"
+[[ ! $diff ]]