Add pytest hooks for importer integration tests

[ericaltendorf]

Migrated with adaptations from v2 codebase.

[Goorzhel]

This patch helped me get unstuck on making my regression tests work on Beangulp.

[johannesjh]

I like the idea of exposing importer tests as pytest/unittest testcases. Regarding the implementation: I think parts of what this pull request contributes exists already in beangulp:

What exists already:

• a CLI for running such regression tests, see beangulp.testing.main
• the logic for these regression tests, compare beangulp.testing._test
• the logic for reading input and expected output, see beangulp.testing._run

What does not exist yet, as far as I see:

• a simple way of exposing beangulp's regressions test functionality to testframeworks such as unittest or pytest. That's what I like about this pull request, I would like that, too.

Question / Suggestion for this pull request:
Would it be better for this pull request to use more of the existing functionality?

[dnicolodi]

Would it be better for this pull request to use more of the existing functionality?

Most definitely. This is the main reason why this stalled. Furthermore, it should also expose the functionality in a way useful also for integration with an unittest test suite, not only with pytest. Unfortunately, I do not have time to work on this myself or to provide detailed guidance on how I think this would be best implemented.