Add tests

The latest refactor resulted in a bunch of tests being dropped. We need to re-add new tests for the following container classes:

• Add SamzaContainer unit tests
• Add TaskInstance unit tests
• Add SerdeManager unit tests
• Add CheckpointManager unit tests
• Add StringSerde unit tests
• Add TaskStorageManager unit tests
• Add DefaultPicker unit tests
• Add SystemConsumers unit tests
• Add SystemProducers unit tests
• Add Util unit tests