There used to be a ton of test assertion helpers in Core like
assert_session, assert_cookie, assert_flash, etc. See
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
I noticed 2 new patches that fall under this category today.
Any of these kinds of helpers should be plugin territory.