Cannot choose what tests to run

Summary

While developing a feature or fixing an issue, it is important to be able to run a single test or a group of tests, without running the entire suite.

The regular ways to do this, for example to run only the frontend tests are:

python3 setup.py test --addopts 'tests/frontend'

Or now that we have a tox layer, also:

tox -e py36 -- tests/frontend

But this has now regressed

Steps to reproduce

Run one of the said commands above to select which tests to run.

What is the current bug behavior?

After running the selected tests, it starts iterating into new tests, starting with paths such as buildstream/plugintestutils/...

What is the expected correct behavior?

That only the selected tests should be run.