add blackbox extension #20

Job	Run time
PHPUnit (ubuntu-latest, 8.2, lowest)	13s
PHPUnit (ubuntu-latest, 8.2, highest)	12s
PHPUnit (ubuntu-latest, 8.3, lowest)	13s
PHPUnit (ubuntu-latest, 8.3, highest)	18s
CS (8.2)	12s
Psalm (8.2)	21s
PHPUnit (ubuntu-latest, 8.4, lowest)	2m 23s
Coverage (ubuntu-latest, 8.2, lowest)	18s
Psalm (8.3)	2m 27s
PHPUnit (ubuntu-latest, 8.4, highest)	13s
Coverage (ubuntu-latest, 8.2, highest)	16s
Psalm (8.4)	2m 27s
PHPUnit (macOS-latest, 8.2, lowest)	3m 21s
Coverage (ubuntu-latest, 8.3, lowest)	18s
PHPUnit (macOS-latest, 8.2, highest)	3m 19s
Coverage (ubuntu-latest, 8.3, highest)	11s
PHPUnit (macOS-latest, 8.3, lowest)	3m 21s
Coverage (ubuntu-latest, 8.4, lowest)	19s
PHPUnit (macOS-latest, 8.3, highest)	3m 13s
Coverage (ubuntu-latest, 8.4, highest)	12s
PHPUnit (macOS-latest, 8.4, lowest)	3m 21s
Coverage (macOS-latest, 8.2, lowest)	1m 12s
PHPUnit (macOS-latest, 8.4, highest)	3m 15s
Coverage (macOS-latest, 8.2, highest)	1m 17s
Coverage (macOS-latest, 8.3, lowest)	1m 8s
Coverage (macOS-latest, 8.3, highest)	1m 22s
Coverage (macOS-latest, 8.4, lowest)	1m 16s
Coverage (macOS-latest, 8.4, highest)	1m 14s
	37m 52s

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