run-tests: fix --debug for .t tests

run-tests: fix --debug for .t tests

When --debug is given to the test runner, run() returns (retcode, None).
Do not try to use None output as a string, and return directly, similarly
as other testers.

--- a/tests/run-tests.py	Mon Nov 15 17:05:54 2010 -0600
+++ b/tests/run-tests.py	Tue Nov 16 10:33:19 2010 +0900
@@ -504,7 +504,8 @@
         vlog("# Running", cmd)
         exitcode, output = run(cmd, options, replacements)
         # do not merge output if skipped, return hghave message instead
-        if exitcode == SKIPPED_STATUS:
+        # similarly, with --debug, output is None
+        if exitcode == SKIPPED_STATUS or output is None:
             return exitcode, output
     finally:
         os.remove(name)