Tests: Add health check thread stall test

This test trigger a "code stall" in a specified thread using the
testpoint mechanism. The testpoint behavior is implemented in
health_stall.c. The testpoint code stall a specific thread processing by
calling sleep(3).

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

The test ensure the threads can be succesfully stalled and that the
health check feature is able to properly detect stalling in non-polling
cases.

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