Go back to Minitest since we can specifiy a version in development and test environments
Use MiniTest in Test Helper

- require MiniTest
- Bring back `assert_equal_html` and mixin to MiniTest
Bring back MiniTest in Tests

- Inherit from `MiniTest::Test`
- Replace `Test::Unit` methods with matching `Minitest` methods
- Refactor `assert_nothing_raised` to use `assert_equal` since
  `Minitest` doesn't define `assert_nothing_raised`
  - See http://blog.zenspider.com/blog/2012/01/assert_nothing_tested.html