TestStringDeduplicationAgeThreshold intermittent failures on Shenandoah

In my GHAs, I see intermittent errors in this test when executing for Shenandoah. Saw them on x64 and x86 Linux, at least.

In all cases, I see something like:

```
Verifying strings: total=10, uniqueFound=10, uniqueExpected=5"
Deduplication not completed, waiting...
```
where it seems Shenandoah interns too much? Also, not sure what the point is in further waiting.

Test then fails with

```
2021-08-03T08:46:38.6203120Z stderr: [Exception in thread "main" java.lang.RuntimeException: String verification failed
2021-08-03T08:46:38.6205296Z at gc.stringdedup.TestStringDeduplicationTools.verifyStrings(TestStringDeduplicationTools.java:220)
2021-08-03T08:46:38.6208132Z at gc.stringdedup.TestStringDeduplicationTools$DeduplicationTest.main(TestStringDeduplicationTools.java:257)
```

This happens when the test runs for -`XX:StringDeduplicationAgeThreshold=15`, but since this is the first threshold tested and the test executes them all sequentially in one subtest maybe all would fail.

Test seem to be new for Shenandoah, or at least executed for Shenandoah since they were split out with JDK-8269032.