testing: clarify that Cleanup is run after subtests complete.

It's good to be explicit, as it's not necessarily obvious (and indeed
the behavior has changed recently with https://go-review.googlesource.com/c/go/+/214822)
without an associated doc comment change).

Change-Id: I99d6398bf15b404b1b1b196e712e926e363251e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/215217


Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
Reviewed-by: Ian Lance Taylor <iant@golang.org>