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
It's because these tests are testing the behavior of `register_template_handler` itself. Isn't it better to explicitly call it in the test, instead of testing something that's hiding behind the helper method?