Why:
* The maintenance scripts in core are mostly untested and testing
the less complex scripts will improve the test coverage in core.
What:
* Add a non-database integration test for purgeMessageBlobStore.php
that mocks MessageBlobStore::clear to just test the maintenance
script.
** Testing that the message cache was actually cleared is not
as simple, and the code being called is already tested
elsewhere. Therefore, mocking the method avoids indirectly
testing already tested code.
Bug: T371167
Change-Id: Ibb16448d2779d34ee8348368f413d821637ecd82
