contrib/distributions: Test code cleanups

- Remove unnecessary test_session() boilerplate when executing eagerly
- Use self.cached_session() instead of self.test_session() when using graphs

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: 211542360