RFR: 8361710: Mark QPathTest as unstable on all platforms

[Kevin Rushforth]

This PR marks QPathTest as "unstable" on all platforms. QPathTest is known to timeout intermittently. See [JDK-8328222](https://bugs.openjdk.org/browse/JDK-8328222). It is already skipped on Linux, since it was causing a lot of noise in our nightly and developer headful test runs. In the last few months, we've seen an increase of failures, especially when running tests with JDK 24 or 25. As a result, I will mark the test as unstable on all platforms by removing the `isLinux()` check.

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Commit messages:
 - 8361710: Mark QPathTest as unstable on all platforms

Changes: https://git.openjdk.org/jfx/pull/1841/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1841&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361710
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1841.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1841/head:pull/1841

PR: https://git.openjdk.org/jfx/pull/1841