build: Remove tests/ stub; make `make test-all` work right

[timmc-edx]

• The tests/__init__.py create/delete thing seems to have been a workaround for something that's no longer relevant, so just remove it.
• The test-all target needed to depend on clean so that build/lib wouldn't stick around and confuse pytest, and the recipe needed to be pruned down to just tox because otherwise we'd run the quality checks twice. (quality is in the tox.ini default env list.)

Merge checklist:

• [x] Version bumped
• [x] Changelog record added
• [x] Documentation updated (not only docstrings)
• [x] Commits are squashed

[timmc-edx]

Good find -- I can remove test_utils as well.