requirements-test.txt: Explicit swh.core[db] as test requirement

instead of pytest-postgresql which is a transitive dependency from swh.core[db]

Related to T2746

requirements-test.txt

 pytest
-pytest-postgresql
 requests_mock
+swh.core[db]
 sqlalchemy-stubs
 swh.scheduler[testing] >= 0.5
 testing.postgresql

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T2746 is now swh/infra/sysadm-environment#2746 (closed)