Add unregister_template_handler to prevent leaks.

A few places in the test codebase had `ActionView::Template.register_template_handler`, but the handlers were not removed after the tests.

This patch adds a new method `unregister_template_handler` to `ActionView::Template::Handlers`, and makes sure related tests are cleaned up properly.

cc @senny
Thanks. I've moved it into `actionview/test/support/template_handler_helper.rb`.