[FIrestore Samples] Update tests to be self-contained and verified for correctness.

[frankyn]

In which file did you encounter the issue?

https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/firestore/cloud-client/snippets_test.py

Describe the issue

Tests aren't self contained and also not being verified for correctness. At the moment, tests only run the samples and verify that they execute without failure. Although, tests do provide some level of support using this method, they aren't verifying state correctness of Firestore post after testing a given sample.

[gguuss]

This may be fixed in the updated tests from @BenWhitehead.

[BenWhitehead]

There are a few older tests that need to have assertions added to them.

[crwilcox]

@BenWhitehead do we have the individual tests we should fix?