[jdk21u-dev] RFR: 8325399: Add tests for virtual threads doing Selector operations

[Goetz Lindenmaier]

I backport this for parity with 21.0.6-oracle.

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Commit messages:
 - Backport 43089bf0065ce85681724d92d26709f4e3ad28ea

Changes: https://git.openjdk.org/jdk21u-dev/pull/998/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=998&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325399
  Stats: 349 lines in 1 file changed: 349 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/998.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/998/head:pull/998

PR: https://git.openjdk.org/jdk21u-dev/pull/998