Add unit tests for CacheConfig

[jordanw-bq]

We should adopt a strategy for testing this class to ensure getting/setting configuration options are sufficiently tested.

Proposed strategy: Similar to Presto, use Airlift's configuration module for verifying configuration behaviour (example: https://github.com/prestodb/presto/blob/master/presto-hive/src/test/java/com/facebook/presto/hive/TestHiveClientConfig.java)