fix thrift/test/py:python_optional_test
fix thrift/test/py:python_optional_test

Summary:
These tests never worked as intended. Whatever `AssertionError` being thrown was caught by the `catch` block and silently ignored.

This diff fixed them properly by using `assertRaises`.

Fixing these no because it's the right behavior.
It's just so that I know I'm doing the right thing when I breaks them in the next diff.

Reviewed By: nanshu

Differential Revision: D18088320

fbshipit-source-id: b1b83f75305aacc2d03c60e976b5188cbe2d7882

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