Fix broken build due to unit test include order

 Commit 4348d1e6f74a87af55c6c01e7a0cb292e9dec114
 introduced a bug that breaks the unit tests unless
 they run in a certain order. Both path.moon
 and os/fs.moon tries to include the same Enum, which
 fails since ffi.cdef can only include definitions once.

 This solves the bug by using Lua variables instead of
 ffi.cdef Enums.