RFR: JDK-8303815: Improve Metaspace test speed

[Thomas Stuefe]

Small adjustments that cut down time needed for Metaspace tests:

gtests: 22 seconds -> 6 seconds
elastic metaspace jtreg tests: 36 seconds -> 15 seconds

For the jtreg tests, I preserve the option to execute them full time as manual tests

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Commit messages:
 - cut down time for elastic metaspace jtreg tests
 - dial down gtests

Changes: https://git.openjdk.org/jdk/pull/15324/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15324&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303815
  Stats: 56 lines in 6 files changed: 34 ins; 5 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/15324.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15324/head:pull/15324

PR: https://git.openjdk.org/jdk/pull/15324