avocado.job: Send SIGUSR1 instead of SIGTERM to test subprocess

Since we are using signals as a form of IPC (inter process
communication), let's use SIGUSR1, which is a user defined
signal that won't be easily mistaken with the more common
SIGTERM, that can be issued far more commonly.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #103 from lmr/test-subprocess-improvements

Test subprocess improvements