Add Unit Test for WindowDataLayer

This PR provides a unit test for WindowDataLayer.

The purpose of this unit test is to protect against issues while the data layer design is improved (such as #1089 and #1125).
@jeffdonahue I replaced the duplicated code in `WindowDataLayerTest` to parse window data file with hard-coded constant variables, and did not explicitly test whether `WindowDataLayer` correctly parses the input file. I only compared the forward results from `WindowDataLayer` and the results I got from `WindowDataLayerTest::load_window_data_and_label`.