[FIX] fastapi: partner env for tests

`partner` record leaks default env and even if you specify
`default_fastapi_running_user` to be custom user, when running tests,
running env will still have default (superuser), which makes testing
outcomes very different than you might expect.

Setting `partner` to use env with specified user, makes sure it won't
leak from there.

fastapi/tests/common.py

@@ -127,6 +127,7 @@ def _create_test_client(
             or self.default_fastapi_authenticated_partner
             or self.env["res.partner"]
         )
+        partner = partner.with_env(env)
         if partner and authenticated_partner_impl in dependencies:
             raise ValueError(
                 "You cannot provide an override for the authenticated_partner_impl "