RFR: 8338060: jdk/internal/util/ReferencedKeyTest should be more robust

[Roger Riggs]

Addressing latent issues with ReferencedKeyTest
- During the `methods()` tests the keys should be strongly held to avoid inadvertent GC collection and subsequent test failures (JDK-8336926)
- Merge changes from Valhalla to use String (identity objects) for keys instead of Integer and Long that are value objects. (JDK-8336390)

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Commit messages:
 - 8338060: jdk/internal/util/ReferencedKeyTest should be more robust

Changes: https://git.openjdk.org/jdk/pull/20527/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20527&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338060
  Stats: 80 lines in 1 file changed: 26 ins; 4 del; 50 mod
  Patch: https://git.openjdk.org/jdk/pull/20527.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20527/head:pull/20527

PR: https://git.openjdk.org/jdk/pull/20527