Restore support for PSR-0-style test-classes (main/10.x)

[totten]

This is a forward-port of #5021 from 9.5 to main/10.x.

[codecov]

Codecov Report

Merging #5030 (a6b188e) into main (0cab10b) will decrease coverage by 0.72%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #5030      +/-   ##
============================================
- Coverage     81.93%   81.21%   -0.73%     
+ Complexity     5700     5643      -57     
============================================
  Files           588      583       -5     
  Lines         14944    14726     -218     
============================================
- Hits          12245    11959     -286     
- Misses         2699     2767      +68     

Impacted Files	Coverage Δ
src/Runner/TestSuiteLoader.php	97.29% <100.00%> (ø)
src/Util/Error/Error.php	0.00% <0.00%> (-100.00%)	⬇️
...Result/Subscriber/TestTriggeredErrorSubscriber.php	0.00% <0.00%> (-100.00%)	⬇️
...esult/Subscriber/TestTriggeredNoticeSubscriber.php	0.00% <0.00%> (-100.00%)	⬇️
...sult/Subscriber/TestTriggeredWarningSubscriber.php	0.00% <0.00%> (-100.00%)	⬇️
src/Event/Dispatcher/CollectingDispatcher.php	0.00% <0.00%> (-50.00%)	⬇️
src/Framework/Exception/Exception.php	64.28% <0.00%> (-35.72%)	⬇️
src/Event/Events/TestRunner/WarningTriggered.php	70.00% <0.00%> (-30.00%)	⬇️
src/Framework/ExceptionWrapper.php	66.66% <0.00%> (-27.28%)	⬇️
src/Util/Error/Handler.php	57.60% <0.00%> (-22.83%)	⬇️
... and 81 more

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[sebastianbergmann]

Thank you!