Add fortytw2/leaktest to all unit tests that involves go routines.

[bwplotka]

Hi, 👋

We found this super useful in the Thanos project, it allows us to find memory leaks in the form of not closed go routines pretty fast. Thanks for @gotjosh who reminds me to put a request for that. 🤗

Help wanted!

NOTE1: It does not work with t.Parallel until this: fortytw2/leaktest#4
NOTE2: At some point, Uber created its own lib which does the same (without even trying to contribute to the existing https://github.com/fortytw2/leaktest) here: https://github.com/uber-go/goleak Since https://github.com/fortytw2/leaktest works well, is mature and have friendly maintainers, I would say let's stick to that. (:

[liguozhong]

good idea！