Add pytest fixture to remove demo bots from sys.modules after use

[Debilski]

While working on #942, I found that the following test fixture can be helpful in allowing us to re-use a module name. (Before, each test would need to use a different file name for test bots, which would make parametrised tests harder.)

@pytest.fixture
def cleanup_test_modules(request):

    # Use a marker like
    #     @pytest.mark.cleanup_test_modules(["test-team-a", "test-team-b"])
    # to override the modules that are cleaned up. Otherwise it defaults to ["test-team"].

    marker = request.node.get_closest_marker("cleanup_test_modules")
    if marker is None:
        modules = ['test-team']
    else:
        modules = marker.args[0]

    for module in modules:
        if module in sys.modules:
            raise RuntimeError(f"Test module {module} is already in sys.modules.")

    yield

    for module in modules:
        del sys.modules[module]

A test can then just add cleanup_test_modules as a parameter and be sure that test-team is not in sys.modules and that it is automatically removed afterwards.

def test_player_import_name(cleanup_test_modules):
    ...
    load_team('test-team.py')
    ...

I’m planning on rewriting the tests to use this, which potentially makes a bunch of tests shorter. This issue is in case there are objections agains this hack.