Fix tests/unix/ffi_float.py to pass on OSX

OSX uses dynamic libraries, so this test needed an entry for ```libc.dylib``` to work.

Side note - I needed to update my locale settings to use ```en-US.UTF-8``` instead of ```en-US.us-ascii``` in order to make the unicode tests pass for CPython.
Now all 293 tests run by run-test.py pass on OSX