tests: use kill -0 to check process existence

When killing a daemon, instead of using kill -9 to check the process was
terminated, use kill -0. Should the pid of the process be reused
immediately after, it would be wrong to kill the new process. Worst case
scenario the kill_daemon function returns before the process is
confirmed to be killed but this is not treated as an error and is
unlikely to cause any problem.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>