add tests for path with time_slice_format (out_file)

`test_out_file` does not test the effect of `time_slice_format` parameter until now. 
This pull request adds tests for `path` parameter with `time_slice_format` parameter. 

To do this, I had to modify `BufferedOutputTestDriver`. I don't know why `test_out_file` is using `BufferedOutputTestDriver` rather than `TimeSlicedOutputTestDriver` though (I actually tried to use `TimeSlicedOutputTestDriver` for `test_out_file`, but it looked the test driver is lacking some of features to test `out_file`).