Unit test not compatible with test.state

Spent some time wrangling with a unit test for a phase that accesses data stored in test.state.

Looks like the unit test framework creates a method local test state `test_state_` that is eventually passed to the phase being unit tested.

https://github.com/google/openhtf/blob/master/openhtf/util/test.py#L183

AFAIK, the unit test author does't have access to this `test_state_`. Would be useful to expose this because some phases rely upon test state (either on test.state or on measurements/attachments).