Improve tests for `SourceMapper` #3102

Job	Run time
Dependency Validation	21s
Coding Guidelines	30s
Static Analysis	31s
Unit Tests (ubuntu-latest, 8.3)	15s
Unit Tests (ubuntu-latest, 8.4)	12s
Unit Tests (ubuntu-latest, 8.5)	11s
Unit Tests (windows-latest, 8.3)	35s
Unit Tests (windows-latest, 8.4)	36s
Unit Tests (windows-latest, 8.5)	32s
End-to-End Tests (ubuntu-latest, 8.3)	42s
End-to-End Tests (ubuntu-latest, 8.4)	42s
End-to-End Tests (ubuntu-latest, 8.5)	40s
End-to-End Tests (windows-latest, 8.3)	1m 48s
End-to-End Tests (windows-latest, 8.4)	1m 49s
End-to-End Tests (windows-latest, 8.5)	1m 45s
Build PHAR	26s
Code Coverage	2m 38s
Test PHAR (8.3, pcov)	21s
Test PHAR (8.3, xdebug)	32s
Test PHAR (8.4, pcov)	22s
Test PHAR (8.4, xdebug)	34s
Test PHAR (8.5, pcov)	17s
Test PHAR (8.5, xdebug)	29s
	16m 48s

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