RFR: JDK-8303605: Memory leaks in Metaspace gtests

[Justin King]

Fix memory leaks identified while running Metaspace gtests.

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Commit messages:
 - Fix various memory leaks in Metaspace tests

Changes: https://git.openjdk.org/jdk/pull/12865/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12865&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303605
  Stats: 23 lines in 4 files changed: 23 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12865.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12865/head:pull/12865

PR: https://git.openjdk.org/jdk/pull/12865