Add tests for last instance session. Fixes #214.

Commits f69ff5f and 832382f didn't add end-to-end test coverage for this
feature. This commit simulates a user experience via the django testing
client.

It also provides a couple of helpers methods for synthesizing urls while
testing.
Merge pull request #217 from ahinz/fix/214/proper-test-coverage

Add tests for last instance session. Fixes #214.