RFR: 8331495: Limit BasicDirectoryModel/LoaderThreadCount.java to Windows only

[Alexey Ivanov]

As soon as I integrated `test/jdk/javax/swing/plaf/basic/BasicDirectoryModel/LoaderThreadCount.java` #18957, the test started failing in CI on Linux and macOS. There are six failures found so far.

Before starting code review and later, I ran the test many times on CI and I never saw so many failures.

To reduce the noise, I'm limiting the test to **Windows only** where it's stable.

I submitted [JDK-8331494](https://bugs.openjdk.org/browse/JDK-8331494) to make the test more stable on Linux and macOS if possible.

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Commit messages:
 - 8331495: Limit BasicDirectoryModel/LoaderThreadCount.java to Windows only

Changes: https://git.openjdk.org/jdk/pull/19038/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19038&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331495
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19038.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19038/head:pull/19038

PR: https://git.openjdk.org/jdk/pull/19038