[tests] Firestore: add asserts into snippets tests

It was noticed that most of the [snippet tests](https://github.com/q-logic/python-docs-samples/blob/f5f3744438e468b4f71cd4524e9f2f1cf7811826/firestore/cloud-client/snippets_test.py) in Firestore doesn't do any asserts.
As per [comments](https://github.com/GoogleCloudPlatform/python-docs-samples/pull/4196#discussion_r451797229) it should be fixed.

/cc @tmatsuo, @busunkim96 