Tests

Exclude first-party stubs from validation #21862

Job	Run time
Integration tests (ubuntu-latest)	10s
Tests (8.2, ubuntu-latest)	9s
Tests (8.3, windows-latest)	9s
Tests with old PHPUnit (7.4, windows-latest)	9s
Tests (8.4, ubuntu-latest)	9s
Tests (8.4, windows-latest)	10s
Integration tests (windows-latest)	10s
Tests with old PHPUnit (8.0, windows-latest)	10s
Tests (8.2, windows-latest)	10s
Determine levels tests matrix	9s
Tests with old PHPUnit (8.1, windows-latest)	9s
Tests with old PHPUnit (7.4, ubuntu-latest)	3s
Tests with old PHPUnit (8.0, ubuntu-latest)	10s
Tests with old PHPUnit (8.1, ubuntu-latest)	9s
Tests (8.3, ubuntu-latest)	10s
Levels tests	1s
	2m 17s

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