conftest: Reference swh.core.db.pytest_plugin

As it's exposed through the swh.storage.pytest_plugin itself used by other swh
modules, this needs to be declared to avoid other swh module build failures.

Related to T2746

conftest.py

@@ -3,4 +3,4 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-pytest_plugins = ["swh.storage.pytest_plugin"]
+pytest_plugins = ["swh.storage.pytest_plugin", "swh.core.db.pytest_plugin"]