Unit Tests and AttachFromFile

New unit test util's are really useful.

Ran into a little bit of a pain point with test.AttachFromFile(filename) since need to mock out open:

```
mo = mock.mock_open()

with mock.patch('openhtf.exe.phase_data.open', mo, create=True):
phase_record = yield MyPhase

assert FILENAME in phase_record.attachments
```

Would be nice if we could make this happen under the hood.