[jdk22u] RFR: 8331142: Add test for number of loader threads in BasicDirectoryModel

[Alexey Ivanov]

Provides a regression test for [JDK-8325179](https://bugs.openjdk.org/browse/JDK-8325179): _Race in BasicDirectoryModel.validateFileCache_ which is backported to 22u.

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Commit messages:
 - Backport b2fb5ea13ba5087411410519213fc953ecc44618

Changes: https://git.openjdk.org/jdk22u/pull/175/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk22u&pr=175&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331142
  Stats: 276 lines in 1 file changed: 276 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk22u/pull/175.diff
  Fetch: git fetch https://git.openjdk.org/jdk22u.git pull/175/head:pull/175

PR: https://git.openjdk.org/jdk22u/pull/175