tools: show stdout/stderr for timed out tests

When a test times out, the contents of stdout and stderr can often be
highly valuable in debugging. Provide that information.

Refs: #19906 (comment)

PR-URL: #20260
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

tools/test.py

@@ -333,7 +333,7 @@ def HasRun(self, output):
       (total_seconds, duration.microseconds / 1000))
     if self.severity is not 'ok' or self.traceback is not '':
       if output.HasTimedOut():
-        self.traceback = 'timeout'
+        self.traceback = 'timeout\n' + output.output.stdout + output.output.stderr
       self._printDiagnostic()
     logger.info('  ...')