**description:** the ChatModel[Integration]Tests classes are powerful
and helpful, this change allows sub-classes to add additional tests.
for instance,
```
class TestChatMyServiceIntegration(ChatModelIntegrationTests):
...
def test_myservice(self, model: BaseChatModel) -> None:
...
```
---------
Co-authored-by: ccurme <chester.curme@gmail.com>
