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
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.
tests: fix whitespace in hash_ordering_test

Indent two spaces, not one.
tests: reset Strainer's filters after modification

Three tests in the test suite use the Liquid::Template.register_filter
function to register custom filters with Liquid::Strainer. The problem
is that these register_filter calls leave the Liquid::Strainer object in
an altered state.

As an example, the FiltersTest's test_local_filter relies on the default
behavior of Liquid::Strainer operator, and the test was failing if
register_function had been called earlier. The same thing was happening
with FiltersInTemplate's test_local_global.

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

  ruby -Itest -e "require 'integration/hash_ordering_test';
  require 'integration/filter_test'"

Update the tests to always reset Liquid::Strainer's filters back to the
default list of filters.

With this change, FiltersTest and FiltersInTemplate now pass.
with_global_filter test helper
Merge pull request #401 from Shopify/ktdreyer-minitest

Minitest 5 (continuation of #358)