x/build/cmd/gopherbot: add Gerrit mock/integration tests

[rolandshoemaker]

We currently have no real tests for the way gopherbot interacts with Gerrit, nor is there a simple way to implement them. For complex (and, arguably, somewhat dangerous functionality, like #48021, autosubmit) this makes implementation a bit of a crapshoot, since we end up mostly just hoping we got the behavior right and testing in prod.

[dmitshur]

Since it's not already mentioned, I'll add that one of the few ways to test gopherbot changes now is via its -dry-run mode. This helps, but it's not a complete nor ideal solution.

If we mock individual Gerrit API responses and create tests based on those, it'll help with some types of changes, but contribute little for types of changes where a sequence of state changes (and behaving exactly like the real Gerrit server does, which changes over time) has important effects on the outcome (as one example, see #21312 (comment)).

There's some similarity and overlap here with #37603, which is about another component maintner that also interacts with Gerrit (and GitHub) (and is one of gopherbot dependencies at this time).

[dmitshur]

git-codereview implements a fake Gerrit server (type gerritServer) and a git helper (type gitTest) for its tests, which might be relevant here (spotted via #50576).