Tests: Add health check thread exit test

This test trigger a failure in a specified thread using the recently
introduced testpoint mechanism. The testpoints behavior is implemented
in health_exit.c. The testpoint code simply calls pthread_exit(3) and
effectively "kill" the thread without affecting the other threads
behavior.

The test select the thread to be "killed" by enabling a specific
environment variable.

With this test we ensure that each thread can be succesfully terminated
and that the health check feature properly detect a failure.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>