Letting `ember test` and `ember nw:test` co-exist

Previously, users could not run ember test when the addon was installed, as it forced everyone to use ember nw:test exclusively. This release ensures users can opt out of running tests in a NW environment by ensuring that the build customizations required for ember nw:test do not get applied when running ember test.

Changes

#36 Fix ember test command (@alexlafroscia)