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.