change(aurweb): add parallel tests and improve aurweb.db

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>

pytest.ini

@@ -8,3 +8,9 @@
 #         https://bugs.python.org/issue45097
 filterwarnings =
     ignore::DeprecationWarning:asyncio.base_events
+
+# Build in coverage and pytest-xdist multiproc testing.
+addopts = --cov=aurweb --cov-append --dist load --dist loadfile -n auto
+
+# Our pytest units are located in the ./test/ directory.
+testpaths = test