[corlib] Fixed test that failed intermittently

There is a race condition in the test, the "#2" Assert() can be reached before e1 is assigned if the thread
that handles e1 gets preempted by the scheduler (this is more likely on single-core systems). Adding a sleep before checking the values fixes this for now.

@marek-safar can you verify this doesn't change the behavior of the test (you added it in b787868fa8553095f2eceacde712768b69b645c4)?