Move from deprecated self.test_session() to self.session() or self.cached_session().

Move to cached_session() if the session is create more than once per test. Move to session() otherwise.

self.test_session() has been deprecated in 9962eb5e as its name confuses readers of the test. Moving to session() instead which slightly changes the semantic of the function:
* the session is not cached anymore (a new session is created).
* the session is closed when exiting the "with" scope.

PiperOrigin-RevId: 217421579