Fix remaining tests and the way we run them
    
    - Drop `-x tests` from %pyproject_buidrequires. It is not needed since
      we had `pytest` as an explicit BR already.
    - Keep `addopts` in `pytest.ini_options` table. It's actually useful for
      running the tests as upstream intended.
    - Simplify %pytest no we have a usable `addopts`.
    - Enable some more tests that work using `--import-mode=importlib`.
    - Remove `tests` from installation after testing (e.g. in %check instead
      of %install). This is a bug in upstreams `pyproject.toml`. Due to the
      flat layout the `exclude` statement becomes meaningless (conflicts the
      `include` statement, which prevails). I'll report that upstream.