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

tools/run-tests.py

Index: tools/run-tests.py
diff --git a/tools/run-tests.py b/tools/run-tests.py
index a9a81f8299878ec09964242ea3ed5169e506c9be..7b4c5c122fd9cca363ae48a2ca2f2d230f7019cf 100755
--- a/tools/run-tests.py
+++ b/tools/run-tests.py
@@ -532,8 +532,6 @@ def Main():
 
 
 def Execute(arch, mode, args, options, suites, workspace):
-  print(">>> Running tests for %s.%s" % (arch, mode))
-
   shell_dir = options.shell_dir
   if not shell_dir:
     if options.buildbot:
@@ -651,6 +649,14 @@ def Execute(arch, mode, args, options, suites, workspace):
   if options.cat:
     return 0  # We're done here.
 
+  if num_tests == 0:
+    print("No tests selected for running!")
+    if len(all_tests) > 0:
+      verbose.PrintReport(all_tests)
+    return 0
+
+  print(">>> Running tests for %s.%s" % (arch, mode))
+
   if options.report:
     verbose.PrintReport(all_tests)