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 If you believe it is necessary to explicitly test whether `WindowDataLayer` is correctly set up and whether the window file is correctly parsed, I think it will involve modifying WindowDataLayer to have `WindowDataLayerTest` as its friend class.