Reference validation and fixtures for QMTests

!1192 (merged) introduced fixtures for QMTests. More specifically, when test B depends on test A, B would only run if A succeeded.

When the output changes for a good reason, A would fail, and a new reference file will be generated. However, B won't run at all. One would have to first update the A reference and then run the tests again.

Instead, we can think about a mechanism to disable fixtures for specific tests like this. (Or other alternatives, such as splitting validation to a reference in another test).

The workaround is to manually remove the fixture for B:

set_property(TEST Subdir.B PROPERTY FIXTURES_REQUIRED)

And run A|B instead of only B when you want to test B