[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#121207

X-original-commit: 4ab8b28b
Signed-off-by: Christophe Simonis <chs@odoo.com>
Signed-off-by: Denis Ledoux (dle) <dle@odoo.com>
Co-authored-by: Alvaro Fuentes <afu@odoo.com>