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 mintiest 5.x, which uses Minitest::Test as
the base test class.

ActiveSupport 4.0 depends on mintiest 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.
Merge pull request #774 from haml/test_case

Add Haml::TestCase