tests: switch to minitest

Ruby 1.9+ uses Minitest as the backend for Test::Unit. As of Minitest 5,
the shim has broken some compatibility with Test::Unit::TestCase in some
scenarios.

Adjusts the test suite to support Minitest 5's syntax.

Minitest versions 4 and below do not support the newer Minitest::Test
class that arrived in version 5. For that case, use the
MiniTest::Unit::TestCase class as a fallback

Conflicts:
	test/integration/tags/for_tag_test.rb
	test/test_helper.rb

test/integration/variable_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class VariableTest < Test::Unit::TestCase
+class VariableTest < Minitest::Test
   include Liquid
 
   def test_simple_variable