Update for Minitest 5.0.x

General updates for Minitest 5.0 compatibility and updates for implicit
subject testing.

* MiniTest -> Minitest
* MiniTest::Unit::TestCase -> Minitest::Test
* Remove anonymous classes for testing the implicit subject
* Add tests for `.must` and `.wont`
* Add true cases for `must` and `wont` with implicit subject

Thanks to @phlipper for his grit and determination. I couldn't have done it without you! :cry: :birthday: