Auto merge of #127968 - fmease:upd-jsondocck-directive-style, r=GuillaumeGomez

Update jsondocck directives to follow ui_test-style

Context: Comment chain in #125813.
Follow-up to #126788.
Use the same temporary approach of "double parsing" until we figure out how we want to support compiletest/ui_test directive "add-ons" for child test runners like HtmlDocCk and JsonDocCk.

I didn't touch much of jsondocck because I want to refactor it some other time (for robustness, maintainability and better diagnostics; basically by following a similar design of my WIP HtmlDocCk-next, cc #125780).

r? `@GuillaumeGomez`

Author: bors