Runtime test to reproduce threadpool+ProcessExit issue

Do make test-process-exit to run
(cherry picked from commit a2e54923bbd578e3fc8bdd95188b017648120de7)
Fixes threadpool+ProcessExit problem

The threadpool was being shutdown too early, before the ProcessExit event was fired.
This led to bugs such as NLog not shutting down
(http://nlog-project.org/2011/10/30/using-nlog-with-mono.html).
(cherry picked from commit 850f2f412499e8f47db7e72bfa80135f9661efff)