Added options to post-review to fill in the Testing field from the command line. 
  --testing=TESTING_DONE
                        details of testing done
  --testing-file=TESTING_FILE
                        text file containing details of testing done

Ran the following:
1) post-review --help to check usage message.
2) Ran with --testing="Ran ran all of the tests" to check specified text appeared in the Testing field of the generated review.
3) Ran with --testing="text" AND --testing-file="/tmp/file" to check mutually exclusive test.
4) Ran with --testing-file="/tmp/does_not_exist" to check exit on missing file.
5) Ran with --testing-file="/tmp/test.txt" to check file content loaded correctly.