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

self.test_session() has been deprecated in 9962eb5e as its name confuses readers of the test. Moving to cached_session() instead which is more explicit about:
* the fact that the session may be reused.
* the session is not closed even when doing a "with self.test_session()" statement.

PiperOrigin-RevId: 213944932