Review Bot never had a real testing framework, leaving us to manually

test any affected functionality. This has been fine historically, as

tools, once written, have generally continued to work fine. However, a

regression was recently found, and it won't be the last, so it's a good

time to finally begin writing tests.
This change introduces a new reviewbot.testing module, with a base

TestCase that containing helpers for creating common objects and diff

data.
It also includes a test runner, and a dev-requirements.txt for

installing the necessary dependencies.
There are no unit tests in this change, but some tests will soon be

provided in another change.

Tested with some upcoming unit tests, making use of all the new

functionality. Tests passed successfully.