Testing generators on an Engine

Hi

I'm writing a generator test for a gem that is supposed to work with
both mountable engines and rails applications. The generator invokes
Rails's model generator.

When running the generator on an engine, it uses Rails model generator
which correctly generates app/models/my_engine/user.rb. But I can't do
it on a test: it doesn't know that it is an engine and run_generator
runs the regular generator which generates app/models/user.rb
instead.

Any hints how to run the generator on "engine mode" in the test ?