Instead of automatically replying with the 'whorl' image for every enroll
state signal with result 'enroll-stage-passed', only perform the number
of required enroll stages and ensure that we get the expected results.
This also will allow to manually perform enroll steps in other tests.
When coverage is enabled fprintd test won't generate any .gcda file and so
apparently no data, this happens because gcov doesn't handle properly the
process termination when SIGTERM is used, and so when in fprintd.py we
terminate the process no coverage data is reported.
To avoid this, quit the main loop cleanly on SIGTERM, so that we will exit
from the main function cleanly, making libc to perform a gcov flush when we
exit the program.
