RFR: 8324799: Use correct extension for C++ test headers

Wed Feb 28 07:06:57 UTC 2024

On Wed, 28 Feb 2024 06:12:17 GMT, Guoxiong Li <gli at openjdk.org> wrote:

> So large patch. In order to easy to review, it is good to separate such large patch into several patches in the future.

I was doing that in prior related changes (see the subtasks of JDK-8324799).  But reviewers requested I batch up
the remainder, hence this PR.

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PR Comment: https://git.openjdk.org/jdk/pull/18035#issuecomment-1968355666