Why:
* Maintenance scripts in core are mostly untested and testing them
will help to avoid regressions.
* Doing this for frequently run scripts is important, as they
may fail in ways not caught by running them in CI.
What:
* Create CheckComposerLockUpToDateTest that fully tests the
associated maintenance script.
** Move test case composer JSON files used by the LockFileCheckerTest
class into the 'phpunit/data' folder instead of placing them
in the same directory to allow the new test class to use these
files too.
Bug: T371167
Change-Id: Id473ef440c6b7f8ecbb1c869f9cc0cf98705a0e9
b380904c66