tortoise.contrib.test.initializer relies on PGDATABASE env var even if db_url is provided

Describe the bug
When supplying a db_url argument to tortoise.contrib.test.initializer which includes the database name
(postgres://user:pass@$host:port/db), it does not respect the DB and instead uses the username supplied as DB name

To Reproduce
Reproducing with:

pytest 8.4.1
tortoise 0.25.1
asyncpg 0.30.0
pytest-asyncio 1.0.0

# conftest.py
@pytest.fixture(scope="session", autouse=True)
def initialize_tests(request):
    db_url = os.environ.get("TEST_DATABASE_URL")

    os.environ["PGDATABASE"] = "dbname"  # <-- removing this should raise:
    # tortoise.exceptions.DBConnectionError: Can't establish connection to default database. Verify environment PGDATABASE

    initializer(["foo.bar"], db_url=db_url)

    request.addfinalizer(finalizer)

Expected behavior
The DB name in the URL should be respected

Additional context
nil

Start at tortoise.contrib.test.initializer and reproduce the issue using the conftest.py fixture and pytest setup shown. Verify that a db_url containing a database name works without PGDATABASE, and run the relevant pytest coverage to confirm the expected connection behavior.