fix: default value for LoopController.continue_forever rather than initializing in the constructor

Previously LoopController initialized continue_forever=true in the constructor,
however, and ThreadGroup.setMainController flipped the value to false.

It caused inconsistency since non-GUI launch does not call ThreadGroup.setMainController.

Using default=true enables to treat "false" as non-standard, so =false is saved
t the jmx, and it gets loaded in non-GUI as well.

Fixes apache#6008

Author: vlsi

src/core/src/main/java/org/apache/jmeter/control/LoopController.java

@@ -48,10 +48,6 @@ public class LoopController extends GenericController implements Serializable, I
 
     private boolean breakLoop;
 
-    public LoopController() {
-        set(getSchema().getContinueForever(), true);
-    }
-
     @Override
     public LoopControllerSchema getSchema() {
         return LoopControllerSchema.INSTANCE;

src/core/src/main/kotlin/org/apache/jmeter/control/LoopControllerSchema.kt

@@ -45,5 +45,5 @@ public abstract class LoopControllerSchema : GenericControllerSchema() {
      * However, it's not clear that a Thread Group could ever be repeated.
      */
     public val continueForever: BooleanPropertyDescriptor<LoopControllerSchema>
-        by boolean("LoopController.continue_forever")
+        by boolean("LoopController.continue_forever", default = true /* previous releases initialized value in constructor */)
 }

xdocs/changes.xml

@@ -109,6 +109,12 @@ Summary
 
 <ch_section>Bug fixes</ch_section>
 
+<h3>Thread Groups</h3>
+<ul>
+  <li><pr>6011</pr>Regression since 5.6: ThreadGroups are running endlessly in non-gui mode: use default value
+    for LoopController.continue_forever rather than initializing it in the constructor</li>
+</ul>
+
 <h3>HTTP Samplers and Test Script Recorder</h3>
 <ul>
 </ul>

xdocs/changes_history.xml

@@ -236,6 +236,8 @@ Apologies if we have omitted anyone else.
 
 <ch_section>Known problems and workarounds</ch_section>
 <ul>
+  <li><issue>6008</issue> ThreadGroups are running endlessly in non-gui mode (fixed in 5.6.1, see <pr>6011</pr>)</li>
+
   <li><pr>5987</pr>HTTP sampler sends filenames with percent-encoded UTF-8, however it is not aligned with the browsers. The workaround is to refrain non-ASCII filenames</li>
 
 <li>The Once Only controller behaves correctly under a Thread Group or Loop Controller,