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.

test/integration/context_test.rb

@@ -16,9 +16,12 @@ class ContextTest < Minitest::Test
       end
     end
 
+    original_filters = Array.new(Strainer.class_eval('@@filters'))
     Template.register_filter(global)
     assert_equal 'Global test', Template.parse("{{'test' | notice }}").render!
     assert_equal 'Local test', Template.parse("{{'test' | notice }}").render!({}, :filters => [local])
+  ensure
+    Strainer.class_eval('@@filters = ' + original_filters.to_s)
   end
 
   def test_has_key_will_not_add_an_error_for_missing_keys

test/integration/filter_test.rb

@@ -111,11 +111,14 @@ class FiltersInTemplate < Minitest::Test
   include Liquid
 
   def test_local_global
+    original_filters = Array.new(Strainer.class_eval('@@filters'))
     Template.register_filter(MoneyFilter)
 
     assert_equal " 1000$ ", Template.parse("{{1000 | money}}").render!(nil, nil)
     assert_equal " 1000$ CAD ", Template.parse("{{1000 | money}}").render!(nil, :filters => CanadianMoneyFilter)
     assert_equal " 1000$ CAD ", Template.parse("{{1000 | money}}").render!(nil, :filters => [CanadianMoneyFilter])
+  ensure
+    Strainer.class_eval('@@filters = ' + original_filters.to_s)
   end
 
   def test_local_filter_with_deprecated_syntax

test/integration/hash_ordering_test.rb

@@ -16,10 +16,13 @@ class HashOrderingTest < Minitest::Test
   include Liquid
 
   def test_global_register_order
+    original_filters = Array.new(Strainer.class_eval('@@filters'))
     Template.register_filter(MoneyFilter)
     Template.register_filter(CanadianMoneyFilter)
 
     assert_equal " 1000$ CAD ", Template.parse("{{1000 | money}}").render(nil, nil)
+  ensure
+    Strainer.class_eval('@@filters = ' + original_filters.to_s)
   end
 
 end