Tests: fix: test_tp_fail: bail out on non-existing relay daemon

Tests: fix: test_tp_fail: bail out on non-existing relay daemon

Fatal thread errors simulated by the tp_fail test cause the relay
daemon to shutdown. This is unexepected by stop_lttng_relayd_notap
which bails out, causing the test to fail.

We bail-out when the daemon is already dead to catch crashes
during the test suite since a0f8e310.

Use the clean-up variant so that we don't fail the tests for this
expected outcome.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I02e7f35451b3c81e7e808e9ff96b6c824fa8f904