[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.

Commit licensed under MIT/X11.

mcs/class/corlib/Test/System/LazyTest.cs

@@ -306,6 +306,8 @@ public void ConcurrentInitialization ()
 				e3 = ex;
 			}
 
+			Thread.Sleep (1000);
+
 			Assert.AreSame (e1, e2, "#2");
 			Assert.AreSame (e1, e3, "#3");
 		}