Enable pylint for tests

Background

Currently we only run pylint on buildstream directory and not tests. There's no real reason why we'd want that except for a few practical limitations around fixtures not playing well, and some other false positives.

I suggest we enable pylint in a more permissive way on tests such that we don't regress again, and avoid re-doing the work that went into MRs like !1195 (merged). Then, we can gradually turn bring the config for tests more inline with the main BuildStream codebase.

Task description

• tests are linted similar to how we lint BuildStream code