Disable Ant tests which don't fork under JDK18

Due to JEP-411, the security manager is being slowly phased out. This results in warnings on JDK17 and runtime exceptions on JDK18. Groovy is already making progress on how to handle that change in other areas. This issue is specifically to disable Ant tests on JDK18 until such time as Ant itself no longer has the issue as per Ant bugzilla issue 65381. If Ant fixes or implements its own workaround mechanism our goal would be to re-enable the tests affected by this issue.