RFR: 7903390: Summary reporter getter not thread-safe

[Jonathan Gibbons]

On Fri, 2 Dec 2022 06:41:42 GMT, Christian Stein <cstein at openjdk.org> wrote:

> Prevent `ConcurrentModificationException` by making the two static methods that populate the instance map `synchronized`.

Ideally we should have a test, but this might be a hard one to test

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Marked as reviewed by jjg (Lead).

PR: https://git.openjdk.org/jtreg/pull/139