RFR: 7903793: Latent typo in ReportOnlyTest.gmk

[Jonathan Gibbons]

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)`

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Commit messages:
 - CODETOOLS-7903793: Latent typo in ReportOnlyTest.gmk

Changes: https://git.openjdk.org/jtreg/pull/222/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=222&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903793
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/222.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/222/head:pull/222

PR: https://git.openjdk.org/jtreg/pull/222