Order the recipients of the instructor email digests

The query which picks the instructors to email had no `ORDER BY`, so
Postgres returned the rows in whatever physical order it found them in,
which depends on what else had been written to `user` before.

That made `test_it_sends_digests_for_instructors` pass or fail by luck: it
asserts the calls come out sorted by `h_userid`, which only held when the
heap happened to be laid out that way. The unit tests all share one
database (`tox.ini` points every worker at `lms_tests`) and run under
`--dist loadgroup`, so which tests a worker picks up — and therefore what
was written to `user` first — depends on the collected test list. Adding
the two dashboard API tests in this branch was enough to reshuffle it and
turn the test red, with no change to the code it covers.

Ordering in the query rather than in the test also makes the order the
digests go out in deterministic in production, which it was not.

Author: gmorador-tribu

lms/tasks/email_digests.py

@@ -100,6 +100,15 @@ def send_instructor_email_digest_tasks():
                         .is_(False)
                     ),
                 )
+                # Without this the rows come back in whatever physical order
+                # Postgres finds them in, which depends on what else has been
+                # written to `user` before. That makes the order the digests go
+                # out in arbitrary, and it makes the test which asserts the
+                # recipients pass or fail depending on which xdist worker picked
+                # it up: the unit tests all share one database, so any change to
+                # the number of tests reshuffles the distribution and can flip
+                # it.
+                .order_by(User.h_userid)
             ).all()
 
             for h_userid in h_userids: