This change utilizes pytest-xdist to perform a multiproc test
run and reworks aurweb.db's code. We no longer use a global
engine, session or Session, but we now use a memo of engines
and sessions as they are requested, based on the PYTEST_CURRENT_TEST
environment variable, which is available during testing.

Additionally, this change strips several SQLite components
out of the Python code-base.

SQLite is still compatible with PHP and sharness tests, but
not with our FastAPI implementation.

More changes:
------------
- Remove use of aurweb.db.session global in other code.
- Use new aurweb.db.name() dynamic db name function in env.py.
- Added 'addopts' to pytest.ini which utilizes multiprocessing.
    - Highly recommended to leave this be or modify `-n auto` to
      `-n {cpu_threads}` where cpu_threads is at least 2.

Signed-off-by: Kevin Morris <kevr@0cost.org>