RFR: 8271834: TestStringDeduplicationAgeThreshold intermittent failures on Shenandoah [v2]

[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.

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Aleksey's comment

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5428/files
  - new: https://git.openjdk.java.net/jdk/pull/5428/files/75eeb7e2..46170eb3

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5428&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5428&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 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