[RFC] Fix luajit cimport method

This PR removes method declarations from `ffi.cdef` in each test, making it easier to begin writing tests for existing functionality. Any `#define` are still required to be cdef'ed as enums for now.

The one thing I am unsure about is the change I had to make to `src/os_unix_defs.h` which was needed to define the `timeval` struct.

I have only tested this on Mac OS X.