gpgscm: Fix handling an error for chdir.
* tests/gpgscm/ffi.c (do_chdir): Use gpg_error_from_syserror.
FWIW, the original idea with gpgscm was to provide code which does no rely on any gpg stuff so it can be merged back into upstream. I am not sure whether this still makes sense.
Background: I encountered a problem error message shows irrelevant; While it should say 'No such file or directly', it says 'Unknown Packet'.