rebar_eunit:reset_after_eunit/1 mustn't remove 'included_applications'

One could argue that rebar_eunit does some pretty dangerous stuff when
trying to clean up after a test run, but specifically, it tells the
Application Controller to delete everything returned by
application:get_all_env(App). Unfortunately, included_applications
also goes, which can lead to a crash in application_controller, if
the application is subsequently unloaded.

This patch attempts the smallest possible fix: remove all env
variables except included_applications.