No longer prevent System.exit() by default

FitNesseRoot/FitNesse/ReleaseNotes/content.txt

@@ -1,4 +1,5 @@
 !2 Pending Changes
+ * IMPORTANT: !style_code[System.exit()] is no longer prevented by default, to allow functioning without issue on JDK > 18. On lower Java versions it can be prevented by setting the system property !style_code[prevent.system.exit] to !style_code[true].
  * Fix SLF4J logging ([[1522][https://github.com/unclebob/fitnesse/pull/1522]])
 
 !2 20240707

FitNesseRoot/FitNesse/SuiteAcceptanceTests/SuiteSlimTests/SystemExitIsPrevented/content.txt

@@ -1,3 +1,5 @@
+This test should be removed or 'pruned' when running on Java 21 or higher, as preventing system exit is no longer supported there.
+
 !|script|SystemExitTableConfiguration |
 |set system property|prevent.system.exit|to|true|
 

src/fitnesse/slim/instructions/SystemExitSecurityManager.java

@@ -54,7 +54,7 @@ private static boolean isPreventSystemExit() {
  if (preventSystemExitString != null) {
  return Boolean.parseBoolean(preventSystemExitString);
  } else {
- return true;
+ return false;
  }
  }
 

test/fitnesse/slim/instructions/SystemExitSecurityManagerTest.java

@@ -1,44 +1,37 @@
 package fitnesse.slim.instructions;
 
-import static org.junit.Assert.fail;
-import static util.RegexTestCase.assertMatches;
-
+import fitnesse.slim.instructions.SystemExitSecurityManager.SystemExitException;
 import org.junit.After;
 import org.junit.Test;
 
-import fitnesse.slim.instructions.SystemExitSecurityManager.SystemExitException;
+import static org.junit.Assert.fail;
+import static util.RegexTestCase.assertMatches;
 
 public class SystemExitSecurityManagerTest {
-
- SecurityManager oldSecurityManager;
-
- SecurityManager securityManager;
-
  @After
  public void teardown() {
  SystemExitSecurityManager.restoreOriginalSecurityManager();
  }
 
  @Test(expected = SystemExitException.class)
  public void shouldThrowExceptionWhenSystemExitIsCalled() {
- acticateSystemExitSecurityManager();
+ activateSystemExitSecurityManager();
  System.exit(0);
  fail("should have thrown exception");
-
  }
 
  @Test
  public void shouldIncludeExitCode() {
  try {
- acticateSystemExitSecurityManager();
+ activateSystemExitSecurityManager();
  System.exit(42);
  fail("should have thrown exception");
  } catch (SystemExitException e) {
  assertMatches("system exit with exit code 42", e.getMessage());
  }
  }
- 
- private void acticateSystemExitSecurityManager() {
+
+ private void activateSystemExitSecurityManager() {
  System.setSecurityManager(null);
  System.setProperty(SystemExitSecurityManager.PREVENT_SYSTEM_EXIT, "true");
  SystemExitSecurityManager.activateIfWanted();