add goroutine leak detector to tests

[miparnisari]

this is purely a safety measure. if these leak detectors fail it could mean a real leak in the code or either a badly designed test that doesn't wait till all goroutines finish before it ends.

i am not sure how to not pollute the main go.mod with this dev- only dependency.

[janos]

Thanks for this check, but I would also like to avoid the dependency. Since currently there are no goroutine leaks, this is a good validation, but does not have to be part of the code since the codebase is pretty stable.