In a previous change, cmd/go was taught to show a "no tests ran" warning
if test did nothing. But it missed a case - if no tests nor examples ran
but any benchmarks were meant to be run, it would still produce the
warning. This meant that running only benchmarks, which is common, would
be confusing:

	 $ go test -run='^$' -bench=.
	testing: warning: no tests to run
	BenchmarkFoo-4            300000              5056 ns/op
	[...]

I believe this was because of a copy-paste error in the tests. This was
being tested, but on the wrong file which does contain a test that was
being run. Fix the path and fix the now failing test by never showing
the warning if -bench was given a non-empty string.

The rationale is that if -bench was given but there was no output, it's
obvious that nothing happened as benchmarks always produce output even
without -v. So showing a warning in those cases is redundant.

To make future typos less likely, make sure that no tests are being run
in the cases where we only want to run benchmarks.

Fixes #17603.

Change-Id: I4c626caf39f72260c6a9761c06446663f465f947
Reviewed-on: https://go-review.googlesource.com/32157Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>