Remove redundant test file.

These haven't been run ever (because the filename doesn't end in
_spec.rb.  Also, the main test in here has no assertions.

This functionality is already covered by end_to_end_spec.rb
Update specs to new rspec syntax

Also change to use rspec's mocking framework instead of mocha.
Port unit tests to rspec.
Remove test_unit tasks and dependencies.
Add some default RSpec config.
Merge pull request #40 from alphagov/unify_tests

Let's use just one test framework.