Follow-up 3270283abf.
This way the test is only run on commits made to this repo, not on all
skins and extensions, and so saving an awful lot of downloads from
happening in CI unnecessarily.
Bug: T203694
Change-Id: I071be5821711fb8b039ff24293f66a8ff9d70514
Enable the equivalent of running `manageForeignResources.php verify`
as a structure test via PHPUnit.
Related cleanups for this to work well:
* Improve cache key readability before enabling in CI.
* Document how we do caching in CI.
* Change some exceptions to errors so that we don't
stop on the first error, thus improving the usefulness of the CI
result by telling the developer all mismatches instead of only
the first.
* Fix incorrect typehint for nullable $integrity.
Bug: T203694
Change-Id: I8aeffc1f2e81ffcf037977528f94c00995791999
