Test cleanups

- Remove unnecessary use of test_session() in tests that run with eager
  execution enabled.
- Use cached_session() instead of test_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: 211562969