Add tests for generated samples

[gkevinzheng]

          One thought: these generated samples aren't covered by tests. Have you considered adding tests for these to catch this kind of issue in the future, like the [other snippets](https://github.com/googleapis/python-logging/tree/c201482c913b5f2eac82359787511a5a92fc3406/samples/snippets)? Smoke tests could be helpful here (i.e. just executing each function and making sure nothing crashes)

There may be a simple way to programmatically load and execute each sample, but if it's bit more complicated we could open a bug for later

Originally posted by @daniel-sanche in #804 (comment)