Reflectometry: Mock filewriter for tests

The reflectometry server autosaves some values to files. Currently, there is no mock version of this filewriter meaning that files are produced sometimes as a side effect of unrelated tests (e.g. changing beamline modes).

This is not ideal since this may lead to tests failing due to file io issues rather than the actual test it is performing. We should have some mechanism for mocking the file writing component out for testing.