Add tests for timeout, and fix a number of resulting bugs.

Summary:
Some of the bugs were in timeout.py, some in alert.py, and some in the
library itself!

Test Plan:
tests/alertlib_test.py
tests/timeout_test.py

Reviewers: chris

Subscribers: tony, alpert

Differential Revision: http://phabricator.khanacademy.org/D8732
Undo our mocking at tearDown() time.

I was being lazy in not doing that!

Test Plan:
tests/alertlib_test.py
tests/timeout_test.py
Add a test for --sender-suffix' to timeout_test.py

Test Plan:
tests/alertlib_test.py
tests/timeout_test.py