[RDY] Fix broken build due to unit tests

Thanks to @stefan991 for tracking this down!

 Commit 4348d1e6f74a87af55c6c01e7a0cb292e9dec114 introduced a bug that breaks the build unless the unit tests run in a certain order. Both path.moon and os/fs.moon tries to include the same Enum, which breaks since ffi.cdef can only include definitions once.

 This solves the bug by using Lua variables instead of ffi.cdef Enums.