Add PHPUnit as dev dependency

Author: ntshcalleia

composer.json

@@ -7,5 +7,8 @@
 		"php" : ">=5.3.0",
 		"codescale/ffmpeg-php" : "2.*",
 		"php-ffmpeg/php-ffmpeg" : "0.11.0"
+	},
+	"require-dev": {
+		"phpunit/phpunit": "^9.5"
 	}
 }