RFR: 8356114: java/foreign/TestBufferStackStress2.java failed with junit action timed out

[Per Minborg]

This PR proposes to skip a stress test if the `main` thread is a virtual thread.

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Commit messages:
 - Skip stress if main is a virtual thread

Changes: https://git.openjdk.org/jdk/pull/25084/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25084&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356114
  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25084.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25084/head:pull/25084

PR: https://git.openjdk.org/jdk/pull/25084