cmd/go, testing: indicate when no tests are run

For example, testing the current directory:

	$ go test -run XXX
	testing: warning: no tests to run
	PASS
	ok  	testing	0.013s
	$

And in a summary:

	$ go test -run XXX testing
	ok  	testing	0.013s [no tests to run]
	$

These make it easy to spot when the -run regexp hasn't matched anything
or there are no tests. Previously the message was printed in the "current directory"
case when there were no tests at all, but not for no matches, and either way
was not surfaced in the directory list summary form.

Fixes #15211.

Change-Id: I1c82a423d6bd429fb991c9ca964c9d26c96fd3c5
Reviewed-on: https://go-review.googlesource.com/22341


Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>