Add Haml::TestCase

Fix Minitest base class warnings by creating a `Haml::TestCase` base
class that inherits from the latest Minitest base class available.

Background:

ActiveSupport 4.1 depends on minitest 5.x, which uses `Minitest::Test` as the base test class.

ActiveSupport 4.0 depends on minitest 4.x, which uses `MiniTest::Unit::TestCase` as the base class.

Rails 3.2 uses `Test::Unit::TestCase` defined by stdlib. There is no gem dependency on minitest in Rails 3.2, nor is there a gem dependency on test-unit.