RFR: CODETOOLS-7903273: Add comment for confusing pattern

[Jonathan Gibbons]

Please review a simple patch to add a comment to some non-obvious code.

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Commit messages:
 - CODETOOLS-7903273: Add comment for confusing pattern

Changes: https://git.openjdk.org/jtreg/pull/108/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=108&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903273
  Stats: 11 lines in 1 file changed: 11 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jtreg/pull/108.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/108/head:pull/108

PR: https://git.openjdk.org/jtreg/pull/108