test,doc: fix lint error in test fixtures

Add first-level header to .md file in fixture directory. The file in
question is documentation for the fixture and is not part of any tests.
This is in prepration for markdown linting of the `test` directory.

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>