run-tests: separate exit code when no failed tests but warned ones

This can be used by buildbot to set the result "warnings" instead of "failed".

tests/run-tests.py

@@ -1167,6 +1167,8 b' def runtests(options, tests):'
 
     if failed:
         return 1
+    if warned:
+        return 80
 
 testtypes = [('.py', pytest, '.out'),
              ('.t', tsttest, '')]