rtapi_app_main_fails tests: make it work on rtai too

This makes the test pass on 2.6-sim, 2.6-rtai, 2.7-uspace,
and 2.7-rtai.

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
better 'make clean'

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>