These are put in their own crate so that we can link tests against libwings freely.
Once libwings is entirely rewritten in Rust, we should be able to move these tests back into the wings-rs crate.
This starts to establish the structure for an in-place rewrite of WINGs. An
actual redesign of WINGs may follow, but for now there are no plans to alter the
structure of WINGs substantially.
Having wings-rs depend on WINGs/WINGs/WINGsP.h was not working very well if all
of WINGs/ had to be built before wings-rs/. Moving wings-rs/ into WINGs/
addresses this.