Write a UnitTest for worker::Job and verify it behaves as expected.

Change-Id: I9f2ee9c1c4d7a5e44f0a796101f720ef146f5b3f
Increase the testing coverage of JobSubmission.

We hadn't test important string conversion helpers.

Change-Id: I820ddf4591c1c5e4b351c933a2504008e4659994