In particular, we were not building these for win32, which has no
symbolic link concept. So let's use $(LN_S) defined by autoconf for this
purpose, as it will copy files for platforms without ln support.
Moreover this way, we don't create one such folder for each and every
test run and fill up the tmp directory with countless directories, that
we never clean up.
On Win32, this fixes 3 tests which were failing because of the missing
icons.