Add test for some aspects of job queue

This new opcode and gnt-debug sub-command test some aspects of the
job queue, including the status of a job. The bug fixed in commit
2034c70d was identified using this test. A future patch will
run this test automatically from the QA scripts.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>