tests: build the test icon theme for all platforms.
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.
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