tests: add test from #3979

Let's add one more test that came up during the discussion of an issue.

The selected name with 69 chars is above the Linux hostname limit of 64.

src/test/test-hostname-util.c

@@ -42,6 +42,7 @@ static void test_hostname_is_valid(void) {
         assert_se(!hostname_is_valid("foo..bar", false));
         assert_se(!hostname_is_valid("foo.bar..", false));
         assert_se(!hostname_is_valid("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", false));
+        assert_se(!hostname_is_valid("au-xph5-rvgrdsb5hcxc-47et3a5vvkrc-server-wyoz4elpdpe3.openstack.local", false));
 
         assert_se(hostname_is_valid("foobar", true));
         assert_se(hostname_is_valid("foobar.com", true));