ActionView::TestUnit

We still don't have an official way to test ActionView Helpers. I noticed a few little patches for ActionController and ActiveSupport TestCases. An ActionView Helper is another obvious fit.

I converted some of my helper modules for testing helpers into a rough patch. It works, but I'm still not happy with the internals.

http://dev.rubyonrails.org/ticket/10056

I definitely like the idea of introducing a class for testing helpers, but I'd probably prefer to wait till after we branch for 2.0

It's been a gap in the test-stack for a while, so it'd be good to let it marinate a bit before it was included in a release.