Force explicit links from UI tests to auxiliary files

[fmease]

Meta: Task: Expand. Maybe something like:

main.lushui:

;;; TEST pass check
;;; TEST auxiliaries aux

aux.lushui

;;; TEST auxiliary main

and that the given auxiliary files

• exists
• have the current test as one of its users

Makes the test suite more robust since erroneous modifying actions (removing/renaming/changing the Lushui code inside of the tests) can potentially be caught: Unreferenced auxiliary file etc.