System.exit tests on Java 18

[remkop]

Many exit-code related picocli tests fail when running on Java 18:

picocli.AutoCompleteTest > testBashRejectsNullScript FAILED
    java.lang.UnsupportedOperationException: The Security Manager is deprecated and will be removed in a future release
        at java.base/java.lang.System.setSecurityManager(System.java:416)
        at org.junit.contrib.java.lang.system.ProvideSecurityManager.before(ProvideSecurityManager.java:39)
        at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:46)
        at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:48)
        at org.junit.contrib.java.lang.system.internal.LogPrintStream$1$1.evaluate(LogPrintStream.java:30)
        at org.junit.contrib.java.lang.system.internal.PrintStreamHandler$3.evaluate(PrintStreamHandler.java:48)
        at org.junit.contrib.java.lang.system.internal.LogPrintStream$1.evaluate(LogPrintStream.java:26)
        at org.junit.contrib.java.lang.system.internal.LogPrintStream$1$1.evaluate(LogPrintStream.java:30)
        at org.junit.contrib.java.lang.system.internal.PrintStreamHandler$3.evaluate(PrintStreamHandler.java:48)
        at org.junit.contrib.java.lang.system.internal.LogPrintStream$1.evaluate(LogPrintStream.java:26)
        at org.junit.rules.RunRules.evaluate(RunRules.java:20)
        ...

• picocli.AutoCompleteTest
• picocli.ExecuteLegacyTest

One issue is that migrated tests cannot be run on Java 5, making it harder to verify that the functionality still works as expected on Java 5. So perhaps only the exit-code tests should be migrated.

Note that the issue can be postponed by setting -Djava.security.manager=allow before running the tests.