Have rule for verifying commit includes tests for new code

[gguuss]

We've had a number of commits into the python-docs-samples repo recently from contributors who are not including tests. It'd be great if we had a bot that enforced coverage / tests for new code.

One approach could be to also use the nox --missingtests functionality.

[tmatsuo]

My 2 cents.

This is reasonably hard to implement this feature for various languages.
Also there are coverage tools which has github integrations for most of the languages.

I think it's better to use such tools. Closing, but feel free to reopen.