[FIX] core: load tests from all entries in `--upgrade-path`

Allow extending tests for any modules, regardless of existing ones
in another entry of the `upgrade-path`.

Bonus point: tests no longer need to be imported in the `__init__.py`
file.

closes odoo/odoo#121150

Signed-off-by: Christophe Simonis <chs@odoo.com>
Co-authored-by: Alvaro Fuentes <afu@odoo.com>