test/db: simplify a bit the handling of test db backends

Rename 'swh/core/test/data/cli' as 'swh/core/test/data/postgresql' so
this directory use the same name as the default db backend
in cli tools (postgresql). This allow to remove a bit of complexity in
mockups for import_swhmodule and get_swh_backend_module.

Also make mock_get_swh_backend_module() depend on
mock_import_swhmodule() since the former need the later to work properly
in most cases.