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
But reading the test I see you explicitly unregister the handler online [401](https://github.com/rails/rails/pull/15590/files#diff-f4614e7ba8081c0c9e20fe64b6374228R401). However the test name suggests that calling render will somehow unregister the handler:

`render unregisteres extension and template handler`

This sentence specifies the behavior of `render` but at the time you are calling `render` the handler was already unregistered. I suggest renaming the test.