tests: Use uatomic for accessing global states

Global states accesses were protected via memory barriers. Use the
uatomic API with the CMM memory model so that TSAN does not warn about
non-atomic concurrent accesses.

Also, the thread id map mutex must be unlocked after setting the new
created thread id in the map. Otherwise, the new thread could observe an
unset id.

Change-Id: I1ecdc387b3f510621cbc116ad3b95c676f5d659a
Co-authored-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>