Ensure templated source tests aren't collected unconditionally
Currently, when attempting to run a subset of the test suite, the templated source tests in
buildstream.testing._sourcetests will be collected, regardless of whether they are desired or not (see #995 (closed))
Changes proposed in this merge request:
- Add some logic to
testing.sourcetests_collection_hookto decide if templated tests should be collected based on the arguments provided to pytest.
This merge request, when approved, will close: #995 (closed)