RFR: 8351000: StringBuilder getChar and putChar robustness

[Andrey Turbanov]

On Tue, 29 Apr 2025 14:10:30 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

> Improve StringBuilder getChar and putChar robustness with racy updates to StringBuilder. 
> Test racing threads with append, insert, and reading characters.

Let's cleanup test a bit

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Changes requested by aturbanov (Committer).

PR Review: https://git.openjdk.org/jdk/pull/24951#pullrequestreview-2806744000