Decode err for tests running under Python 3.

PiperOrigin-RevId: 240813313

testing/run_tests.py

@@ -35,6 +35,7 @@ def main():
       print('**** %s ****\n' % test)
       proc = subprocess.Popen(['python', '-m', test], stderr=subprocess.PIPE)
       _, err = proc.communicate()
+      err = err.decode()
       print(err)
       failed = FAILED_RE.search(err)
       if failed: