librariangen: run tests on pull requests

[jskeet]

We should run the librariangen unit tests in a GitHub action (or as part of the existing Kokoro job) to find issues as part of normal pull requests etc

[jskeet]

@quartzmo: It's been a while since I created this issue; I created a PR for it at the time, but we decided not to create a new GHA... I think we probably still need to fix things to make sure that the tests are run.

I've just created #13288 with a deliberately failing test, to see whether that's caught. If it is, we can close this issue. If not, we should figure out why.

[jskeet]

That PR is all green, so I think this really does need some attention after all.

[jskeet]

Note that the librariangen-specific aspect will become obsolete when librariangen code moves to the librarian repo, but I'll leave this open for now in case it's symptomatic of a bigger failure to test.