RFR: 8326831 Clarify test harness control variables in make help

[Guoxiong Li]

On Tue, 27 Feb 2024 13:55:53 GMT, Ludvig Janiuk <lujaniuk at openjdk.org> wrote:

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

> I have no idea what causes it.

Nevermind. The patch looks good.

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Marked as reviewed by gli (Committer).

PR Review: https://git.openjdk.org/jdk/pull/18028#pullrequestreview-1907859906