tests: switch to minitest 5

The first commit in this pull request switches Liquid's test suite 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.)

The second commit in this pull request fixes an issue with IfElseTagTest clobbering the original operator in Liquid and leaving it in an altered state. This commit fixes a test failure in ConditionUnitTest's test_contains_works_on_arrays, which relies on the original behavior of the "contains" operator.