lisa_shell: Return nosetests' exit code from lisa-test
lisa_shell: Return nosetests' exit code from lisa-test

This allows CI tools to easily detect whether anything went wrong

bjackman/lisa,ARM-software/lisa,joelagnel/lisa,mdigiorgio/lisa,credp/lisa,arnoldlu/lisa,ARM-software/lisa,credp/lisa,mdigiorgio/lisa,ARM-software/lisa,bjackman/lisa,joelagnel/lisa,arnoldlu/lisa,credp/lisa,ARM-software/lisa,credp/lisa