avocado.utils.process: Add a little more debug info to the SubProcess class

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
selftests/functional: Add back interrupt behavior test

Although not strictly correct, simulate attempts to
interrupt avocado via SIGINT to the parent avocado
process. This will trigger the message pressed on
the 2nd Ctrl+C, and further SIGINT will kill the
process.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>