Fix: Re-enable final_class fixer

[localheinz]

This PR

• re-enables the final_class fixer
• runs make coding-standards
• uses the real FixtureFactory in tests

Follows #1.

[codecov]

Codecov Report

Merging #33 into master will decrease coverage by 0.63%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master      #33      +/-   ##
============================================
- Coverage     61.85%   61.22%   -0.64%     
- Complexity       80       81       +1     
============================================
  Files             8        8              
  Lines           194      196       +2     
============================================
  Hits            120      120              
- Misses           74       76       +2

Impacted Files	Coverage Δ	Complexity Δ
src/FieldDef.php	0% <ø> (ø)	6 <0> (ø)	⬇️
src/EntityDef.php	0% <ø> (ø)	18 <0> (ø)	⬇️
src/FixtureFactory.php	91.56% <66.66%> (-2.27%)	37 <12> (+1)

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