[jdk21u-dev] RFR: 8331142: Add test for number of loader threads in BasicDirectoryModel

[Andrew Lu]

I backport this for parity with 21.0.5-oracle.

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Commit messages:
 - Backport b2fb5ea13ba5087411410519213fc953ecc44618

Changes: https://git.openjdk.org/jdk21u-dev/pull/725/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=725&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331142
  Stats: 277 lines in 1 file changed: 277 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/725.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/725/head:pull/725

PR: https://git.openjdk.org/jdk21u-dev/pull/725