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

[Coleen Phillimore]

On Wed, 29 Oct 2025 18:54:09 GMT, Leonid Mesnik <lmesnik at openjdk.org> wrote:

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

This seems quite reasonable.

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Marked as reviewed by coleenp (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28048#pullrequestreview-3525928981