[System.Runtime.Caching] Make TestExpiredGetValues test more reliable

It failed twice on Jenkins over the weekend:

```
Test Case Failures:
1) MonoTests.System.Runtime.Caching.MemoryCacheTest.TestExpiredGetValues :   #EGV4
  Expected: 0
  But was:  9

at MonoTests.System.Runtime.Caching.MemoryCacheTest.TestExpiredGetValues () [0x0012b] in /media/ssd/jenkins/workspace/test-mono-mainline/label/debian-armel/mcs/class/System.Runtime.Caching/Test/System.Runtime.Caching/MemoryCacheTest.cs:1304
```

We're sleeping just as long as the expiry time of the cache items (1s) which might not be enough.
Bumping the sleep time should fix this.

mcs/class/System.Runtime.Caching/Test/System.Runtime.Caching/MemoryCacheTest.cs

@@ -1,4 +1,4 @@
-//
+//
 // MemoryCacheTest.cs
 //
 // Authors:
@@ -1293,7 +1293,7 @@ namespace MonoTests.System.Runtime.Caching
 
 				Assert.AreEqual (10, mc.GetCount (), "#EGV2");
 
-				global::System.Threading.Thread.Sleep (1000);
+				global::System.Threading.Thread.Sleep (4 * 1000);
 
 				// we have waited but the items won't be expired by the timer since it wont have fired yet
 				Assert.AreEqual (10, mc.GetCount (), "#EGV3");