Add a linter to GitLab Pages to force the use of goleak in all tests

Add a linter to GitLab Pages to force the use of goleak in all tests.

The following discussion from !1013 (merged) should be addressed:

• @arun.sori started a discussion: (+7 comments)

  We have found some Goroutine leaks in unit tests.

  OK so the intent is to force the use of goleak in all tests? If so, I wonder if there are other linters out there so that we don't have to implement our own. If not, perhaps that linter should live in labkit instead of eventually copying to this repo as we would have to maintain it twice.