Fix TestCase | only "models" works as app_label

    - in the current implementation only "models" works as app_label
    - there is no way to change that during run time as it's hard coded
    - the fix introduces another _APP_LABEL global variable that is set
       in the initializer

tortoise/contrib/test/__init__.py

@@ -43,6 +43,7 @@
 _LOOP: AbstractEventLoop = None  # type: ignore
 _MODULES: Iterable[Union[str, ModuleType]] = []
 _CONN_CONFIG: dict = {}
+_APP_LABEL = None
 
 
 def getDBConfig(app_label: str, modules: Iterable[Union[str, ModuleType]]) -> dict:
@@ -103,7 +104,9 @@ def initializer(
     global _TORTOISE_TEST_DB
     global _MODULES
     global _CONN_CONFIG
+    global _APP_LABEL
     _MODULES = modules
+    _APP_LABEL = app_label
     if db_url is not None:  # pragma: nobranch
         _TORTOISE_TEST_DB = db_url
     _CONFIG = getDBConfig(app_label=app_label, modules=_MODULES)
@@ -247,7 +250,7 @@ class IsolatedTestCase(SimpleTestCase):
 
     async def _setUpDB(self) -> None:
         await super()._setUpDB()
-        config = getDBConfig(app_label="models", modules=self.tortoise_test_modules or _MODULES)
+        config = getDBConfig(app_label=_APP_LABEL, modules=self.tortoise_test_modules or _MODULES)
         await Tortoise.init(config, _create_db=True)
         await Tortoise.generate_schemas(safe=False)
 
@@ -327,7 +330,7 @@ class TestCase(TruncationTestCase):
 
     async def asyncSetUp(self) -> None:
         await super().asyncSetUp()
-        self._db = connections.get("models")
+        self._db = connections.get(_APP_LABEL)
         self._transaction = TransactionTestContext(self._db._in_transaction().connection)
         await self._transaction.__aenter__()  # type: ignore