We used to support the `BACKTRACE` environment variable but when
we switched to MiniTest it got removed: f9382cd794
This commit adds back the functionality to show the unfiltered backtrace when needed.
This also works when you run your tests with `rake`:
* `BACKTRACE=1 bin/rake test`
* `BACKTRACE=1 ruby -Itest ...`
This removes all deprecated classes in ActionController related to
Routing, Abstract Request/Response and Integration/IntegrationTest.
All tests and docs were changed to ActionDispatch instead of ActionController.