This introduces a new TestCase class and a command line tool for easily

unit testing Review Board extensions.
The new ExtensionTestCase can be used by extension authors to create

unit tests that make use of an enabled instance of the extension. This

makes use of Djblets's new testing support, and also allows for the

usage of the handy object creation methods we have in Review Board's

main TestCase class.
A new rbext test command is now provided, which makes it easy to run

an extension's test suite. This works similarly to other test runners

like nosetests, but sets up a suitable Review Board environment in which

to run the tests.
To make that work well, and to consolidate code, the main Review Board

test runner now makes use of Djblets's new TestRunner class. This means

that both Review Board and any extensions will be running in the same

kind of environment.
Documentation has been added describing how to make use of all this to

write and run test suites.

Ran the Review Board test suite.
Updated an extension to use ExtensionTestCase, and then ran the

test suite using rbext test. I also tried a variety of other

options (built-in and through nosetests), and tried running

subsets of tests. All worked.
Read through the new docs and verified that the formatting and

linking was all correct.