Make it clearer that -test=X/Y runs all the tests matching X,
even if they don't have sub-tests matching Y.

Fixes #20589.

Change-Id: Ic27e89e748d60f67b50c68445ec0480066bdf207
Reviewed-on: https://go-review.googlesource.com/46030
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>