testing: add test helpers for measuring goroutine leaks

[bradfitz]

It's common for tests to check for goroutine leaks, but the testing package or standard
library doesn't make this easy.

These tests end up verbose and often frail.

We should provide something.

[rsc]

Comment 1:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 2:

Labels changed: added repo-main.

[akalin-keybase]

@bradfitz You reference this bug from #12989 saying that you were working on it. Has anything come out of that? :)

[bradfitz]

I do have a package partially written for this, but I never got happy enough to send it out and found more important things to do. I think I'll put it somewhere outside of the standard library first. I'll update this bug if/when I do.