[Go] advice about testing/synctest

Reference: https://github.com/golang/go/issues/67434

In the cases where it is necessary for testing, historically (inside Google) gophers have been advised to pass around values such as `func() time.Time`, or some similar fake-able clock, instead of calling `time.Now()`. With synctest it seems like a new strategy is to simply rely on the new primitives rather than bothering with that plumbing.

Does the Google Go style guide have an opinion on this?

(@matttproud @gaal)