RFR: 8365919: Replace currentTimeMillis with nanoTime in Stresser.java

[Paul Hohensee]

On Thu, 21 Aug 2025 10:02:12 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Mostly a mechanic change from `currentTimeMillis()` to `nanoTime()/1000000`. It also removes `finishTime` to avoid overflowing.
> 
> The change in `iteration()` is to ensure `currentTime` is properly initialized.
> 
> (Was investigating a timeout on Windows-x64, which led me to this code. I think this fix is good enough by its own.)
>  
> Test: tier1-5

You might want to load (options.getTime() * 1000) into a local rather than compute it multiple times.

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PR Review: https://git.openjdk.org/jdk/pull/26879#pullrequestreview-3145246364