Tests: use SIGKILL to shutdown daemons in test_thread_exit and test_tp_fail

A current design limitation of the lttng-consumerd will cause it to
hang on shutdown if the timer management thread exits as the teardown
of channels switches off the channel's timers. The timer thread is
then expected to purge timer signals and signal when it is done.

Obviously this state will never be reached as signals are no longer
being processed. This is not dramatic as this is not what this test
is meant to test; we only want to make sure the health check signals that
something went wrong.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>