tests: Run filter-redirector and -mirror test only on POSIX systems

This way we can get rid of the ugly #ifdefs in the code which makes
it easier to extend later.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <1502951113-4246-2-git-send-email-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>