RFR: 8326831 Clarify test harness control variables in make help

[Ludvig Janiuk]

Clarifying text in `make help` output on how to list variables for e.g. JTREG='...'.

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Commit messages:
 - 8326831 Clarify test harness control variables in make help

Changes: https://git.openjdk.org/jdk/pull/18028/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18028&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326831
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18028.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18028/head:pull/18028

PR: https://git.openjdk.org/jdk/pull/18028