Add warnings to JUnit report - closes #3793

[DanielRuf]

This should properly add the warnings to the output of the JUnit log, see #3793.

[codecov]

Codecov Report

Merging #3910 into 8.4 will increase coverage by 1.18%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                8.4    #3910      +/-   ##
============================================
+ Coverage     83.68%   84.86%   +1.18%     
- Complexity     3853     5155    +1302     
============================================
  Files           151      166      +15     
  Lines         10199    12954    +2755     
============================================
+ Hits           8535    10994    +2459     
- Misses         1664     1960     +296

Impacted Files	Coverage Δ	Complexity Δ
src/Util/Log/JUnit.php	90.32% <100%> (+0.32%)	34 <0> (ø)	⬇️
src/Runner/StandardTestSuiteLoader.php	54.66% <0%> (-12.91%)	27% <0%> (+16%)
src/Framework/MockObject/Invocation.php	87.37% <0%> (-2.9%)	52% <0%> (+22%)
.../Framework/MockObject/Builder/InvocationMocker.php	92% <0%> (-1.75%)	50% <0%> (+17%)
src/Util/Printer.php	49.15% <0%> (-0.85%)	33% <0%> (+13%)
src/Runner/TestSuiteSorter.php	100% <0%> (ø)	71% <0%> (+10%)	⬆️
src/Framework/TestBuilder.php	100% <0%> (ø)	41% <0%> (+15%)	⬆️
src/Framework/TestSuiteIterator.php	100% <0%> (ø)	11% <0%> (ø)	⬇️
src/Util/XdebugFilterScriptGenerator.php	100% <0%> (ø)	7% <0%> (ø)	⬇️
...rc/Framework/MockObject/Matcher/InvokedAtIndex.php	57.14% <0%> (ø)	6% <0%> (?)
... and 24 more

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