RFR: 8370905: Update vm.defmeth tests to use virtual threads

[Leonid Mesnik]

The vm.defmeth stress tests should use
`TestThreadFactory.newThread()`
to start virtual threads if test thread factory is used.

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Commit messages:
 - 8370905: Update vm.defmeth tests to use virtual threads

Changes: https://git.openjdk.org/jdk/pull/28048/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28048&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370905
  Stats: 14 lines in 1 file changed: 4 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/28048.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28048/head:pull/28048

PR: https://git.openjdk.org/jdk/pull/28048