Make it easier to report errors from srctree tests by adding the last output...

Make it easier to report errors from srctree tests by adding the last output to the reported AssertionError.

runtests.py

@@ -1946,13 +1946,15 @@ class EndToEndTest(unittest.TestCase):
             cmd.append(command)
             out.append(_out)
             err.append(_err)
+
             if res == 0 and b'REFNANNY: ' in _out:
                 res = -1
             if res != 0:
                 for c, o, e in zip(cmd, out, err):
                     sys.stderr.write("%s\n%s\n%s\n\n" % (
                         c, self._try_decode(o), self._try_decode(e)))
-            self.assertEqual(0, res, "non-zero exit status")
+                self.assertEqual(0, res, "non-zero exit status, last output was:\n%r\n-- stdout:%s\n-- stderr:%s\n" % (
+                    ' '.join(command), self._try_decode(out[-1]), self._try_decode(err[-1])))
         self.success = True