Add database backend methods to get hardcoded or nonexistent primary key values for tests

[Tim Graham]

While hardcoding primary key values in the Django test suite is avoided as much as possible, sometimes there are legitimate uses. The existing hardcoded integer values, however, don't work on databases that don't use integer primary key values. For example, MongoDB uses DEFAULT_AUTO_FIELD = "django_mongodb_backend.fields.ObjectIdAutoField".

I propose adding new methods DatabaseOperations.get_hardcoded_pk() and get_nonexistentd_pk() to allow a database backend to provide suitable values, minimizing the size of the Django fork needed for running the test suite on such databases.