Why:
* The WikimediaMessages extension currently has no tests.
* This is not ideal, considering that the extension is deployed on
WMF production and can be run on every page load (such as
through the ::onBeforePageDisplay hook).
* Testing the extension makes the code easier to maintain
and more reliable to regressions.
Why:
* Add some unit tests that cover a few easy to cover areas of
Hooks.php.
* This is done separately to make it easier to see the coverage
change in future patches.
Bug: T315774
Change-Id: I916668ae0375e8668b09a16111d83bf2e2feee40