gc/stringdedup/TestStringDeduplicationInterned.java fails with Shenandoah

The test gc/stringdedup/TestStringDeduplicationInterned.java fails when run with Shenandoah as the GC. (Shenandoah is currently not in the `@requires` list of supported collectors for this test, to disable running this test until this issue has been dealt with.)

Snippet from the log is

End: Full GC 3/3
Waiting for deduplication...
[10.468s][info ][gc ] Cancelling GC: Stopping VM
];
 stderr: [Exception in thread "main" java.lang.RuntimeException: Deduplication has not occurred
at gc.stringdedup.TestStringDeduplicationTools$InternedTest.main(TestStringDeduplicationTools.java:297)

This may be a testbug rather than an issue with Shenandoah.