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

Committed: https://crrev.com/1b1de2d221a0ed23e529e91bf9fa3dc287acb2ca
Cr-Commit-Position: refs/heads/master@{#30095}

[Jakob Kummerow, 2015-08-08 22:01:38]

jkummerow@chromium.org changed reviewers:
+ jkummerow@chromium.org

[Jakob Kummerow, 2015-08-08 22:01:39]

lgtm

[Michael Achenbach, 2015-08-10 09:17:22]

lgtm

[adamk, 2015-08-10 18:14:08]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2015-08-10 18:14:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1283513003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1283513003/1

[commit-bot: I haz the power, 2015-08-10 18:16:29]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-10 18:16:39]

Patchset 1 (id:??) landed as
https://crrev.com/1b1de2d221a0ed23e529e91bf9fa3dc287acb2ca
Cr-Commit-Position: refs/heads/master@{#30095}

[Michael Achenbach, 2015-08-10 18:54:32]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1283593005/ by machenbach@chromium.org.

The reason for reverting is: Some bots that don't run tests (by design) don't
return the test results json anymore which makes the infrastructure side fail
now..

[adamk, 2015-08-10 19:02:37]

Sorry for the breakage, this one should have gone without NOTRY=true I guess.