testing: move frontend desymref test to custom format

[superlopuh]

It's not very clear from the diff, but here are the changes:

1. remove the outer builtin.module and dedent
2. move check lines above inputs
3. move everything to custom syntax

[compor]

Thanks, we need this in many places.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.25%. Comparing base (5934e29) to head (d618da8).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3813   +/-   ##
=======================================
  Coverage   91.25%   91.25%           
=======================================
  Files         461      461           
  Lines       57557    57557           
  Branches     5549     5549           
=======================================
  Hits        52523    52523           
  Misses       3609     3609           
  Partials     1425     1425           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.