When using test -cover or -coverprofile the output for "no test files"
is the same format as for "no tests to run".

Fixes #24570

Change-Id: If05609411676d42d94c1feac4bc839974fae2cc1
Reviewed-on: https://go-review.googlesource.com/115095
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>