tools: include exit code in test failures

Include the exit code in test failures. This will give us more
information during the currently-puzzling failures that provide no
information in CI such as:

```
03:10:10 not ok 563 parallel/test-fs-truncate
03:10:10   ---
03:10:10   duration_ms: 1.119
03:10:10   severity: fail
03:10:10   stack: |-
03:10:10   ...
```

tools/test.py

@@ -284,7 +284,7 @@ def HasRun(self, output):
     if output.UnexpectedOutput():
       status_line = 'not ok %i %s' % (self._done, command)
       self.severity = 'fail'
-      self.traceback = output.output.stdout + output.output.stderr
+      self.traceback = "exit code: " + output.output.exit_code + "\n" + output.output.stdout + output.output.stderr
 
       if FLAKY in output.test.outcomes and self.flaky_tests_mode == DONTCARE:
         status_line = status_line + ' # TODO : Fix flaky test'