Engine: add tests for ParseJob()

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Engine: tiny cosmetic fix

To make @creack happy :)

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #4315 from shykes/engine-parsejob-test

Engine: add tests for ParseJob()