Use minitest instead of test-unit.

test-unit doesn't give us the ability to skip tests.

Also bump mocha for compatibility with newer versions.
Port classes and includes to minitest versions.
Port `assert_not_<x>` methods to `refute_<x>`.
Rewrite tests that used `assert_nothing_raised`.

minitest deliberately doesn't support this, because "it's not
useful enough" <http://www.ruby-forum.com/topic/493248>.
Skip the admin wrapper integration tests.

They're currently so unpredictable as to be harmful, and since
we're working on a better way to handle settings updates, this
problem should be short-lived.
Merge pull request #61 from alphagov/skip-integration-tests

Skip the admin wrapper integration tests