Fixup adding mock path to UserCommands

[teutoburg]

As was pointed out by @hugobuddel in #627 (comment), using relative paths here is generally wrong.

Also, using patch_fake_symlinks on the mock path can create undesired results.

Also, over in the tests, the fixture needs to be applied on a function level.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.74%. Comparing base (d793e2c) to head (7ce187a).
Report is 281 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #640   +/-   ##
=======================================
  Coverage   76.74%   76.74%           
=======================================
  Files          70       70           
  Lines        8576     8576           
=======================================
  Hits         6582     6582           
  Misses       1994     1994           

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