Add tests for StringHelper

[jawira]

I wrote more tests for \Phing\Util\StringHelper

[codecov]

Codecov Report

Merging #1603 (d1e176e) into main (16ae1f0) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #1603      +/-   ##
============================================
+ Coverage     56.76%   56.79%   +0.02%     
  Complexity     6908     6908              
============================================
  Files           354      354              
  Lines         16733    16733              
============================================
+ Hits           9499     9503       +4     
+ Misses         7234     7230       -4     

Impacted Files	Coverage Δ	Complexity Δ
src/Phing/Util/StringHelper.php	100.00% <0.00%> (+11.76%)	21.00% <0.00%> (ø%)

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