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.

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.