tools,build: apply markdown linting to test dir

Enable markdown linting of the test directory. This change is applied
only to Makefile and not vcbuild.bat because we do not currently lint
anything outside of the doc directory using vcbuild.bat. In the
Makefile, the other targets are called "misc" but that feature does not
currently exist in vcbuild.bat. Adding it would be good, but outside the
scope of this change.

PR-URL: https://github.com/nodejs/node/pull/22221


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>