Override routeset for controller specs

Had to override rspec-rails `routes=` because I couldn't figure other
way to set the routes for all controller specs.

Previously the build used the `use_route` approach which isn't really
the proper supported way to run engine controller specs (current rails
engines guides is misleading)

This should also fix false positives on api controller specs

Because the build used the :use_route key, tests would pass even when
there would be no route configured for the particular request

there's some conversation going on about the rails issue here
https://github.com/rails/rails/pull/17453