TestPortableRunner should use JobServerDriver interface

Currently, TestPortableRunner uses a pseudo-interface that expects certain methods from an Object. It should instead be expecting the real JobServerDriver interface.

https://github.com/apache/beam/blob/75e3986171ce68a6fc4c42689698576c407f05b1/runners/reference/java/src/main/java/org/apache/beam/runners/reference/testing/TestPortableRunner.java#L68