avocado.job: Fix Ctrl+C handling (for real this time)

1) Revert the test subprocess side SIGINT handler
   to its original form (just raising an appropriate
   TestInterruptError exception)
2) Move the KeyboardInterrupt handling to the test
   run main loop, as we had nested handling blocks
   that ended up creating a deadlock.
3) Start counting the ignore window time after Ctrl+C is
   pressed by the second time, not the first.

With this, we finally get trinity to call its Ctrl+C
handler correctly and cleanup after itself, as well
as avocado cleaning up correctly if enough time is
given to the test subprocesses to end.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
tests/trinity: Don't print ResultCmd object

Not necessary or desirable, since we get trinity's
output in the logs already, and the output of the
test is quite extensive.

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