Clarify job ID-related type checks, add unittests

Instead of a rather complicated expression only “JobId” is output. Job
ID lists (like generated by “SubmitManyJobs”) are limited to two-item
lists. Unittests are added.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>