Fix: Run php-cs-fixer for fixtures

[localheinz]

This PR

• runs php-cs-fixer for fixtures on GitHub actions as well
• fixes an import following Enhancement: Use ergebnis/php-cs-fixer-config #145
• disables the final_public_method_for_abstract_class fixer for fixtures
• disables the constant_case fixer for fixtures

[codecov]

Codecov Report

Merging #148 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #148   +/-   ##
=========================================
  Coverage     84.18%   84.18%           
  Complexity      135      135           
=========================================
  Files            22       22           
  Lines           449      449           
=========================================
  Hits            378      378           
  Misses           71       71

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