[jdk21u-dev] RFR: 8325255: jdk.internal.util.ReferencedKeySet::add using wrong test

[Aleksey Shipilev]

Clean backport, follow up for recent [JDK-8310913](https://bugs.openjdk.org/browse/JDK-8310913) integration. Nothing AFAICS is using the affected method except the test itself.

Additional testing:
 - [x] New regression test fails without the fix, passes with it

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Commit messages:
 - Backport a7461de231da20ed78a7c6ad8275f1bafbfa556c

Changes: https://git.openjdk.org/jdk21u-dev/pull/557/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=557&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325255
  Stats: 33 lines in 3 files changed: 32 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/557.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/557/head:pull/557

PR: https://git.openjdk.org/jdk21u-dev/pull/557