Look at adding helpers for integration tests

As of recent versions of Rails, they've begun discouraging the use of `ActionController::TestCase` for new tests and encouraging the use of integration tests based around `ActionDispatch::IntegrationTest`. The helpers in this gem are still based around the previous style; we may need to look at implementing a second set of helpers to enable the use of integration tests.

See: https://api.rubyonrails.org/classes/ActionController/TestCase.html