Doc: Mention that tests must be run with --keepdb

[MichaelAkvo]

After the introduction of project hierarchies (#4710) using ltree, once the db is created, tests can't delete it and rerun migrations. ltree is an extension that has to be activated and in order to do that, root permissions are required.

The database creation and extension activation are done in docker entrypoint scripts that have postgres root access. Therefore, --keepdb becomes mandatory.