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
tests: reset "contains" op during IfElseTagTest

Two tests in IfElseTagTest each set a custom operator function for the
"contains" comparison operator.

The problem is that IfElseTagTest was clobbering the original operator
in Liquid and leaving it in an altered state.

As an example, ConditionUnitTest's test_contains_works_on_arrays relies
on the specific behavior of the "contains" operator, and its
test_contains_works_on_arrays was failing.

The problem was present when both test classes were require'd inside a
single ruby process. One example is "rake test", which runs "require" on
every test file. Another basic example is the following command:

  ruby -Itest -e "require 'integration/tags/if_else_tag_test.rb';
  require 'unit/condition_unit_test.rb'"

This would cause test_contains_works_on_arrays to fail.

Update IfElseTagTest to avoid clobbering the "contains" operator.

With this change, ConditionUnitTest's test_contains_works_on_arrays now
passes.