Draft: Add a directory for non-source test files

Some tests might require pre-made files. For example, I would like to have a few .omwaddon files to test my serialization/query MR. I'm sure it will be useful for testing various other parts of OpenMW.

I'm not sure my CMake changes are exactly what we need though. Specifically, I don't really understand the Apple-specific parts, and whether we need them for the test app.