Convert Xelib unit tests to run with SkyrimSE data, and remove need for test esps

[leontristain]

This would formalize SkyrimSE as the platform for all tests, such that people who want to invoke tests would only need SkyrimSE installed.

Also remove the need to manually copy esps into the data folder. Instead, I can create those ESPs dynamically during test setup as part of the test, and delete them on teardown.