Drop xinetd for SLE 15, leap 15 and TW

xinetd was dropped and should not be scheduled for leap 15, sle 15 and
TW. See [poo#32674](https://progress.opensuse.org/issues/32674).

lib/main_common.pm

@@ -1208,7 +1208,8 @@ sub load_yast2_ncurses_tests {
     # TODO https://progress.opensuse.org/issues/20200
     # softfail record #bsc1049433 for samba and xinetd
     loadtest "console/yast2_samba";
-    loadtest "console/yast2_xinetd";
+    # xinetd was dropped for sle 15, leap 15 and TW
+    loadtest "console/yast2_xinetd" if is_sle('<15') || (is_leap && !leap_version_at_least('15.0'));
     loadtest "console/yast2_apparmor";
     loadtest "console/yast2_lan_hostname";
     # internal nis server in suse network is used, but this is not possible for