Fix: Mark object as immutabke

[localheinz]

This pull request

• marks an object as immutable

[codecov]

Codecov Report

Merging #114 (e91aa89) into main (0d4617d) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main     #114      +/-   ##
============================================
- Coverage     99.05%   99.04%   -0.01%     
  Complexity       59       59              
============================================
  Files            14       14              
  Lines           211      209       -2     
============================================
- Hits            209      207       -2     
  Misses            2        2              

Impacted Files	Coverage Δ
src/Formatter/ToMillisecondsDurationFormatter.php	100.00% <ø> (ø)
src/Subscriber/TestPassedSubscriber.php	92.59% <0.00%> (-0.52%)	⬇️

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