Use pytest-xdist to run tests

[StevenMaude]

Depends on #2495, and should be rebased when #2495 is merged.

It's possible to run the tests with pytest-xdist since each test is now independently runnable. This makes the unit tests run about 10 seconds or so quicker in CI (about 20-30% faster over not running with pytest-xdist). I see a similar improvement locally.

[StevenMaude]

This might need some checking with Playwright too, though in principle we should make those tests run independently too. (Playwright supports xdist; it's a question of us writing our tests to also on separate workers without any interdependencies.)

[StevenMaude]

I tried this out with the Playwright tests, and it's highlighted something slightly odd there.

They mostly work, but the test_build_bnf_codelist_single_search doesn't seem to work when run with two workers (but does when run with one):

Traceback (most recent call last):
  File "…venv/opencodelists-3.12/lib/python3.12/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
  File "…venv/opencodelists-3.12/lib/python3.12/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…venv/opencodelists-3.12/lib/python3.12/site-packages/django/contrib/auth/decorators.py", line 60, in _view_wrapper
    return view_func(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…local/src/github.com/opensafely-core/opencodelists/opencodelists/views/user_create_codelist.py", line 27, in user_create_codelist
    return handle_post(request, user, owner_choices)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…local/src/github.com/opensafely-core/opencodelists/opencodelists/views/user_create_codelist.py", line 43, in handle_post
    return handle_post_valid(request, form, user, owner_choices)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…local/src/github.com/opensafely-core/opencodelists/opencodelists/views/user_create_codelist.py", line 82, in handle_post_valid
    codelist = create_codelist_from_scratch(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…pythons/lib/python3.12/contextlib.py", line 81, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "…local/src/github.com/opensafely-core/opencodelists/codelists/actions.py", line 198, in create_codelist_from_scratch
    cache_hierarchy(version=version)
  File "…local/src/github.com/opensafely-core/opencodelists/codelists/actions.py", line 654, in cache_hierarchy
    hierarchy = version.calculate_hierarchy()
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…local/src/github.com/opensafely-core/opencodelists/codelists/models.py", line 420, in calculate_hierarchy
    return self._calculate_new_style_hierarchy()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…local/src/github.com/opensafely-core/opencodelists/codelists/models.py", line 432, in _calculate_new_style_hierarchy
    return Hierarchy.from_codes(self.coding_system, list(code_to_status))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…local/src/github.com/opensafely-core/opencodelists/codelists/hierarchy.py", line 44, in from_codes
    ancestor_relationships = set(coding_system.ancestor_relationships(codes))
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…local/src/github.com/opensafely-core/opencodelists/coding_systems/bnf/coding_system.py", line 68, in ancestor_relationships
    return query(sql, codes, database=self.database_alias)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…local/src/github.com/opensafely-core/opencodelists/opencodelists/db_utils.py", line 8, in query
    with connections[database].cursor() as c:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…venv/opencodelists-3.12/lib/python3.12/site-packages/django/test/testcases.py", line 198, in __call__
    raise DatabaseOperationForbidden(self.message)
django.test.testcases.DatabaseOperationForbidden: Database queries to 'bnf_test_20200101' are not allowed in this test. Add 'bnf_test_20200101' to pytest_django.fixtures._django_db_helper.<locals>.PytestDjangoTestCase.databases to ensure proper test isolation and silence this failure.

FAILED opencodelists/tests/functional/test_build_codelist.py::test_build_bnf_codelist_single_search[non_organisation_login_context-asthma]
FAILED opencodelists/tests/functional/test_build_codelist.py::test_build_bnf_codelist_single_search[organisation_login_context-asthma]

I'm not quite sure why, because at least the BNF fixture being used is listed in the Playwright test (although this is about setting up the fixture, so maybe there's a issue of timing when this applies there?)

However, all the tests do run independently: if you assign the same number of workers as tests, so every test runs in a different worker, all the Playwright tests pass.