RFR: JDK-8275608: runtime/Metaspace/elastic/TestMetaspaceAllocationMT2 too slow

[Thomas Stuefe]

Small tweaks to make these elastic metaspace stress tests faster.

Before, runtime for individual tests was wobbling between 40+ seconds and some rare timeouts at 3+ minutes, depending on machine load and random luck. Now, we get a steadfast 35-37 seconds.

I tested this manually, checking the numbers that we really test what we want to test (a lot of arena activity). Also GHAs and SAP nightlies.

Thanks, Thomas

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Commit messages:
 - tweaks to improve runtime

Changes: https://git.openjdk.java.net/jdk/pull/6041/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6041&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275608
  Stats: 27 lines in 4 files changed: 16 ins; 3 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6041.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6041/head:pull/6041

PR: https://git.openjdk.java.net/jdk/pull/6041