[System.Runtime.Caching] Bump timeout in tests to make them more reliable

[System.Runtime.Caching] Bump timeout in tests to make them more reliable

Failed on Jenkins on ARM today:

```
Tests run: 38, Failures: 1, Not run: 0, Time: 32.132 seconds

Test Case Failures:
1) MonoTests.System.Runtime.Caching.MemoryCacheTest.TimedExpiration :   #A4-4
  Expected: not null
  But was:  null

at MonoTests.System.Runtime.Caching.MemoryCacheTest.TimedExpiration () [0x002b6] in /media/ssd/jenkins/workspace/test-mono-mainline/label/debian-armel/mcs/class/System.Runtime.Caching/Test/System.Runtime.Caching/MemoryCacheTest.cs:902
at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00038] in /media/ssd/jenkins/workspace/test-mono-mainline/label/debian-armel/mcs/class/corlib/System.Reflection/MonoMethod.cs:295
```

100ms could be too short, bumping the timeout should make it more reliable on the slower ARM machines.