ConcurrentModificationException in TestMethodController

### Apache NetBeans version

Apache NetBeans 16

### What happened

probably related to #4608

```
java.util.ConcurrentModificationException
at java.base/java.util.ArrayList.forEach(ArrayList.java:1513)
at org.netbeans.modules.gsf.testrunner.ui.api.TestMethodController.setTestMethodsImpl(TestMethodController.java:67)
at org.netbeans.modules.gsf.testrunner.ui.api.TestMethodController.lambda$setTestMethods$0(TestMethodController.java:44)
at org.netbeans.editor.GuardedDocument.runAtomic(GuardedDocument.java:296)
at org.openide.text.NbDocument.runAtomic(NbDocument.java:411)
at org.netbeans.modules.gsf.testrunner.ui.api.TestMethodController.setTestMethods(TestMethodController.java:43)
at org.netbeans.modules.java.testrunner.ui.ComputeTestMethodsImpl$TaskImpl.lambda$run$0(ComputeTestMethodsImpl.java:106)
at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1418)
at org.netbeans.modules.openide.util.GlobalLookup.execute(GlobalLookup.java:45)
at org.openide.util.lookup.Lookups.executeWith(Lookups.java:278)
[catch] at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:2033)
```

### How to reproduce

- Create new JFrame Form component
- Create another new JFrame Form component
- For each form do as follows:
- In the navigator window select the root element (Form)
- In the properties window deselect "Generate Full Classnames" option
- click on source tab
- delete the remaining fully classified names one by one (e.g. javax.swing.JFrame -> JFrame) and add the impor using hint
- save changes and get back to design mode

### Did this work correctly in an earlier version?

No / Don't know

### Operating System

Windows 10 version 10.0 running on amd64; UTF-8; en_US (nb)

### JDK

OpenJDK 64-Bit Server VM 18.0.2+9-61

### Apache NetBeans packaging

Apache NetBeans provided installer

### Anything else

Product Version: Apache NetBeans IDE 16
Java: 18.0.2; OpenJDK 64-Bit Server VM 18.0.2+9-61
Runtime: OpenJDK Runtime Environment 18.0.2+9-61
System: Windows 10 version 10.0 running on amd64; UTF-8; en_US (nb)

### Are you willing to submit a pull request?

No

### Code of Conduct

Yes

Start with TestMethodController.setTestMethodsImpl at line 67 and follow the call from ComputeTestMethodsImpl.TaskImpl.run shown in the stack trace. Reproduce the two-JFrame workflow and verify that it no longer raises ConcurrentModificationException while updating test methods.

Written by the indexing model from the issue text.