tests: restrict cp --link inode comparisons to compatible systems

tests: restrict cp --link inode comparisons to compatible systems

* tests/cp/link-deref.sh: On systems were cp can't determine if
gnulib linkat() emulation might create a symlink instead of a hardlink
to a symlink, copy.c will create a symlink to the symlink so that
it has more control over its metadata.  Also even if the system
supports this operation, the particular file system under test may not.
So avoid the hardlinked symlink verification in these cases.
This fixes a false failure on aix, solaris and freebsd.