Add something equivalent to QMTEST_DISABLE_FIXTURES_REQUIRED for pytest tests

The following discussion from !1605 (merged) should be addressed:

• @clemenci started a discussion:

  In LHCb we use a CMake flag to switch from FIXTURE_REQUIRED to DEPENDS, so that, for special builds, we can ignore failures in the tests dependency chain (see !1465 (merged)).

  8ba85b9b allows something similar via the environment variable GAUDI_TEST_IGNORE_STDOUT_VALIDATION=1, but the tests are artificially marked as passed even if they failed.

  It will be useful to restore the feature from !1465 (merged), may be via a runtime environment variable (made possible by the new way of collecting the tests).