Examine the tox4 plugin tests and make sure all tox3 tests are covered

[hroncok]

This is a follow-up for https://github.com/fedora-python/tox-current-env/pull/42#issuecomment-1348919856

Ideally we,

• [ ] make sure all relevant tests from tox3 are present in tox4
• [ ] write a metatest that ensures this (optional)
• [ ] deduplicate tests that can be identical (e.g. test_passenv) to a new test file that runs with both versions of tox