Clarify test harness control variables in make help

"make help" outputs

  ...
  JTREG="OPT1=x;OPT2=y" # Control the JTREG test harness, use 'help' to list

The "use 'help' to list" text is not straightforward to follow. Common incorrect interpretations are:

  make help JTREG
  make help JTREG=help

The correct way to list those options is:

  [configure]
  make test-only JTREG=help

Let's write that out explicitly in the help.