[jdk11u-dev] RFR: 8331142: Add test for number of loader threads in BasicDirectoryModel

[Andrew Lu]

I backport this for parity with 11.0.25-oracle.
The test is limit to Windows, so backport with JDK-8331495 together.

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Commit messages:
 - Backport b2fb5ea13ba5087411410519213fc953ecc44618
 - Backport b2fb5ea13ba5087411410519213fc953ecc44618
 - Backport b2fb5ea13ba5087411410519213fc953ecc44618

Changes: https://git.openjdk.org/jdk11u-dev/pull/2778/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2778&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/jdk11u-dev/pull/2778.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2778/head:pull/2778

PR: https://git.openjdk.org/jdk11u-dev/pull/2778