Fix broken build due to unit test include order

 Commit 4348d1e6f74a87af55c6c01e7a0cb292e9dec114
 introduced bug that broke the build unless the
 unit tests ran in a certain order. Both path.moon
 and os/fs.moon tried to include the same Enum, which
 breaks since ffi.cdef can only include definitions once.

 The bug is solved by using Lua variables instead of
 ffi.cdef Enums.