Tests rely on unspecified row order of SELECT without ORDER BY

[Yaroslav K]

Many (~70) tests assert a specific row order for querysets whose SQL has no
ORDER BY. SQL standard does not define that order.

On PostgreSQL the rows usually arrive in physical storage order, which
on a freshly loaded table matches insertion order -- so these assertions
pass by coincidence rather than by contract. That breaks when rows are
updated or deleted and the space is reused, or when the planner picks a
different access path such as an index scan. A test like this can start
failing with no change to Django or to the test itself.

Proposed solution:

1. add order_by() where the asserted sequence is meaningful
2. use assertCountEqual() otherwise

Found by running the suite under
​pg_disorder.