[System.Runtime.Caching] Fixed test that failed intermittently on slow systems

Apparently on slow systems the items with 1sec expiration time are already removed from the cache by the time the #CS3 Assert() is hit, which caused the test to fail.
Increasing the expiration time by +2sec should hopefully fix this.

Commit licensed under MIT/X11.
Merge pull request #1113 from akoeplinger/fix-cache-test

[System.Runtime.Caching] Fixed test that failed intermittently on slow systems