java.util.ConcurrentModificationException while generating Testlists

**Summary**

Test bucket parent jobs (e.g. Test_openjdk21_hs_extended.openjdk_s390x_linux) can fail with an intermittent CME while the Testlists are being created.

**URLs**

- Example: https://ci.adoptium.net/job/Test_openjdk21_hs_extended.openjdk_s390x_linux/129/
- History: November 2025: https://github.com/adoptium/temurin-build/issues/4305
- History: October 2025: https://github.com/adoptium/aqa-tests/issues/6664

**Error message**

```
00:13:29.237 java.util.ConcurrentModificationException
00:13:29.237 at java.base/java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:756)
00:13:29.237 at java.base/java.util.LinkedHashMap$LinkedKeyIterator.next(LinkedHashMap.java:778)
00:13:29.237 at java.base/java.util.AbstractCollection.addAll(AbstractCollection.java:335)
```

Start with the example CI job, Test_openjdk21_hs_extended.openjdk_s390x_linux/129, and inspect its full log around Testlist generation and the LinkedHashMap stack trace. Read the linked historical issues in temurin-build#4305 and aqa-tests#6664; the issue does not specify files or acceptance criteria, so done would require identifying the concurrent modification and confirming the intermittent failure no longer occurs.

Written by the indexing model from the issue text.