Comment on PRs if a remote call to merge a change failed

This adds a comment to PRs when a successfully tested change fails
to merge into an upstream repository due to a Github API error. It
uses the merge_failure_message to format said message.

Change-Id: Ieed299a71133ebd88e2c31d6bd0c54390c0901ba
Signed-off-by: Adam Gandelman <adamg@ubuntu.com>
diff --git a/tests/unit/test_github_driver.py b/tests/unit/test_github_driver.py
index f287790..c8e7341 100644
--- a/tests/unit/test_github_driver.py
+++ b/tests/unit/test_github_driver.py
@@ -335,6 +335,8 @@
         self.fake_github.emitEvent(D.getCommentAddedEvent('merge me'))
         self.waitUntilSettled()
         self.assertFalse(D.is_merged)
+        self.assertEqual(len(D.comments), 1)
+        self.assertEqual(D.comments[0], 'Merge failed')
 
     @simple_layout('layouts/dependent-github.yaml', driver='github')
     def test_parallel_changes(self):