RFR: 7903793: Latent typo in ReportOnlyTest.gmk

[Iris Clark]

On Mon, 19 Aug 2024 22:34:44 GMT, Jonathan Gibbons <jjg at openjdk.org> wrote:

> Please review a test-only fix for a mistake in `ReportOnlyTest.gmk`.
> 
> The report directory previously had a double prefix of the value of `$(BUILDTESTDIR)`

Marked as reviewed by iris (Reviewer).

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PR Review: https://git.openjdk.org/jtreg/pull/222#pullrequestreview-2246702624