spec: fix fragile redis delayed_job specs

Jobs on separate strands inserted at very close to the same time might
get processed in a different order, due to the way that redis uses
timestamps for ordering rather than database ids.

This can also change which of the two calls in some specs return the
job, and which don't, after manual redis manipulation.

Change-Id: I2029ccd4ddd273a98b9bb7fdc19ccfc5cfd9e488
Reviewed-on: https://gerrit.instructure.com/33733
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>