add coverage for loader

[romaninsh]

testing new coverage by adding test for a loader.

[codecov]

Codecov Report

Merging #263 into develop will increase coverage by 1.31%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop     #263      +/-   ##
=============================================
+ Coverage      71.34%   72.66%   +1.31%     
  Complexity      1110     1110              
=============================================
  Files             62       62              
  Lines           2729     2729              
=============================================
+ Hits            1947     1983      +36     
+ Misses           782      746      -36

Impacted Files	Coverage Δ	Complexity Δ
src/App.php	79.11% <0%> (+1.77%)	93% <0%> (ø)	⬇️
src/View.php	82.22% <0%> (+2.53%)	129% <0%> (ø)	⬇️
src/Callback.php	96.66% <0%> (+6.66%)	10% <0%> (ø)	⬇️
src/Loader.php	91.66% <0%> (+91.66%)	9% <0%> (ø)	⬇️

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