Improve performance of `NamePrettifier::prettifyTestMethodName()` #821

Job	Run time
Coding Guidelines	46s
Dependency Validation	11s
Type Checker	29s
Unit Tests (ubuntu-latest, 8.1)	13s
Unit Tests (ubuntu-latest, 8.2)	10s
Unit Tests (ubuntu-latest, 8.3)	10s
Unit Tests (ubuntu-latest, 8.4)	17s
Unit Tests (windows-latest, 8.1)	38s
Unit Tests (windows-latest, 8.2)	36s
Unit Tests (windows-latest, 8.3)	54s
Unit Tests (windows-latest, 8.4)	45s
End-to-End Tests (ubuntu-latest, 8.1)	35s
End-to-End Tests (ubuntu-latest, 8.2)	49s
End-to-End Tests (ubuntu-latest, 8.3)	37s
End-to-End Tests (ubuntu-latest, 8.4)	37s
End-to-End Tests (windows-latest, 8.1)	1m 39s
End-to-End Tests (windows-latest, 8.2)	1m 39s
End-to-End Tests (windows-latest, 8.3)	1m 34s
End-to-End Tests (windows-latest, 8.4)	1m 48s
Code Coverage	1m 17s
Build PHAR	37s
Test PHAR (8.1, pcov)	20s
Test PHAR (8.1, xdebug)	54s
Test PHAR (8.2, pcov)	25s
Test PHAR (8.2, xdebug)	38s
Test PHAR (8.3, pcov)	17s
Test PHAR (8.3, xdebug)	31s
Test PHAR (8.4, pcov)	18s
Test PHAR (8.4, xdebug)	32s
	20m 16s

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