Automate tests for STT on prem

[dandhlee]

STT on prem snippets were added on GoogleCloudPlatform/python-docs-samples#4223 without automated tests. Being internally tracked on b/161897398. After some discussion we should add a mock unit test first and then follow up with a more integrated test when there is resource to do so.

[meredithslota]

Closing this issue here since it's tracked internally and the samples have fully migrated to python-docs-samples (I can't transfer issues between orgs, unfortunately).