RFR: 8271834: TestStringDeduplicationAgeThreshold intermittent failures on Shenandoah

[Zhengyu Gu]

To trigger young GC, the test fills heap with -Xmn amount of garbage. However, -Xmn does not apply to non-generational GC, such as Shenandoah, and -Xmn amount of garbage, sometimes, it not enough to trigger GC, therefore, there is no guarantee that strings can reach deduplication age threshold.

I purpose to use MXBean to get exact gc count to ensure expected gc cycles actually performed.

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Commit messages:
 - 8271834: TestStringDeduplicationAgeThreshold intermittent failures on Shenandoah

Changes: https://git.openjdk.java.net/jdk/pull/5428/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5428&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271834
  Stats: 49 lines in 1 file changed: 43 ins; 2 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5428.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5428/head:pull/5428

PR: https://git.openjdk.java.net/jdk/pull/5428