Add composer test entry point and php parallel lint test

This is so the composer-test Job can be added in Jenkins:
 https://gerrit.wikimedia.org/r/#/c/239873/

Change-Id: I77163d2dc8f20f985fd55d59f2e9c5ffdb14a57e

composer.json

@@ -20,5 +20,13 @@
 	},
 	"suggest": {
 		"mediawiki/cldr": "Language names in all languages"
+	},
+	"require-dev": {
+		"jakub-onderka/php-parallel-lint": "0.9.*"
+	},
+	"scripts": {
+		"test": [
+			"parallel-lint . --exclude vendor"
+		]
 	}
 }