tools: fix type mismatch in test runner

`output.diagnostic` is a list that is appended to on SmartOS when
retrying a test due to `ECONNREFUSED`. The test runner checks if
`output.diagnostic` is truthy and, if so, assigns its value to
`self.traceback`. However `self.traceback` is supposed to be a string,
and `_printDiagnostic()` in the `TapProgressIndicator` attempts to call
`splitlines()` on it, which fails if it is a list with:
AttributeError: 'list' object has no attribute 'splitlines'

tools/test.py

@@ -375,7 +375,7 @@ def HasRun(self, output):
 
       if output.diagnostic:
         self.severity = 'ok'
-        self.traceback = output.diagnostic
+        self.traceback = '\n'.join(output.diagnostic)
 
 
     duration = output.test.duration