run-tests.py: warn when no tests were run
run-tests.py: warn when no tests were run

The previous approach to this (in 1b1de2d22) did not work well with
bots that intentionally run no tests. But I think this warning message
is still useful for developers.

Review URL: https://codereview.chromium.org/1281313004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30116}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh