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

The previous approach to this (in 1b1de2d2) 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: refs/heads/master@{#30116}

tools/run-tests.py

@@ -697,6 +697,10 @@ def Execute(arch, mode, args, options, suites, workspace):
 
   if options.time:
     verbose.PrintTestDurations(suites, overall_duration)
+
+  if num_tests == 0:
+    print("Warning: no tests were run!")
+
   return exit_code