There used to be a ton of test assertion helpers in Core like assert_session, assert_cookie, assert_flash, etc. See http://dev.rubyonrails.org/browser/trunk/actionpack/lib/action_controller/assertions/deprecated_assertions.rb?rev=4935 for some history. I wasn't around for the extract reason they were removed, but we don't need a helper method for every possible situation in Rails. And assert @response.flash[:notice] isn't that bad.
I noticed 2 new patches that fall under this category today.
http://dev.rubyonrails.org/ticket/8971 http://dev.rubyonrails.org/ticket/8973
Any of these kinds of helpers should be plugin territory.