Update minitest-rails integration

minitest-rails now replaces the Rails test case classes instead of
existing alongside them, so Keynote's MiniTest base class is now
Keynote::TestCase instead of Keynote::MiniTest::TestCase.
Update other test integrations with new API

Instead of generating test files with an example call to
`FooPresenter.new(view, ...)`, we now offer an implementation of
present/k for test cases and update the generated files accordingly.