tests: fix test-icmp6-rs

Don't close the fd given to sd-icmp6-nd, since it will be aynschonously closed
by sd_icmp6_nd_unref

src/libsystemd-network/test-icmp6-rs.c

@@ -136,7 +136,6 @@ static void test_rs(sd_event *e) {
         nd = sd_icmp6_nd_unref(nd);
         assert(!nd);
 
-        close(test_fd[0]);
         close(test_fd[1]);
 }