In jdk21u-dev testing, caught a test failure with Shenandoah:

```
$ CONF=linux-x86_64-server-fastdebug make images test TEST=java/lang/runtime/ReferencedKeyTest.java TEST_VM_OPTS="-XX:+UseShenandoahGC"
TEST: java/lang/runtime/ReferencedKeyTest.java

java.lang.ClassCastException: class java.lang.runtime.SoftReferenceKey cannot be cast to class java.lang.runtime.WeakReferenceKey (java.lang.runtime.SoftReferenceKey and java.lang.runtime.WeakReferenceKey are in module java.base of loader 'bootstrap')
        at java.base/java.lang.runtime.ReferencedKeyMap.removeStaleReferences(ReferencedKeyMap.java:326)
        at java.base/java.lang.runtime.ReferencedKeyMap.put(ReferencedKeyMap.java:209)
        at java.base/java.lang.runtime.ReferencedKeyTest.populate(ReferencedKeyTest.java:106)
        at java.base/java.lang.runtime.ReferencedKeyTest.mapTest(ReferencedKeyTest.java:61)
        at java.base/java.lang.runtime.ReferencedKeyTest.main(ReferencedKeyTest.java:45)
        at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
        at java.base/java.lang.reflect.Method.invoke(Method.java:580)
        at com.sun.javatest.regtest.agent.MainWrapper$MainTask.run(MainWrapper.java:138)
        at java.base/java.lang.Thread.run(Thread.java:1583)
```

I actually suspect this is a test bug, as it looks like we are leaking the references of one type to another test through the reference queue.