RFR: 7903931: Add "jtreg.ttf" property to filter tests based on test thread factory value

[Christian Stein]

On Tue, 14 Jan 2025 04:06:37 GMT, Leonid Mesnik <lmesnik at openjdk.org> wrote:

> jtreg support test thread factory parameter that allows to execute tests by running main in virtual thread.
> 
> Currently, all incompatible tests are problemlisted. However after security manager removal the number of such tests is small.
> 
> So is it makes sense to add "jtreg.ttf" property to filter tests based on test thread factory value to allow filter out incompatible tests.
> 
> The only jtreg knows if -testThreadFactory is set so it should set corresponding property

What about re-using the `test.` namespace used by `jtreg` for [system properties](https://openjdk.org/jtreg/tag-spec.html#testvars)? Meaning, we'd call this property introduced here:
- `test.threadFactory`

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PR Comment: https://git.openjdk.org/jtreg/pull/242#issuecomment-2618235168