Update tests to use QCNs for fixture classes

[phansys]

Originally, the motivation for using constants for the fixture classes was the lack of a proper reference to these classes.
Now, we can use the ::class pseudo constant with the QCNs instead of its hardcoded string representation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.84%. Comparing base (0632ab1) to head (c3a8c2a).
Report is 65 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2854      +/-   ##
==========================================
+ Coverage   78.75%   78.84%   +0.09%     
==========================================
  Files         163      167       +4     
  Lines        8593     8638      +45     
==========================================
+ Hits         6767     6811      +44     
- Misses       1826     1827       +1     

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