Make run-tests.py warn when it's not testing anything

Make run-tests.py warn when it's not testing anything

I've often been confused by an "all passed" run of the test script
when it turned out that either all the tests I cared about were skipped
or, more likely, I mistyped the name(s) of the tests I wanted to run.
This patch aims to fix that (and gives a useful diagnostic for the
"all matched tests were skipped" case).

R=machenbach@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1283513003

Cr-Commit-Position: refs/heads/master@{#30095}