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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.82%. Comparing base (0632ab1) to head (d552a2a). Report is 77 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2854      +/-   ##
==========================================
+ Coverage   78.75%   78.82%   +0.07%     
==========================================
  Files         163      167       +4     
  Lines        8593     8637      +44     
==========================================
+ Hits         6767     6808      +41     
- Misses       1826     1829       +3     

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