add phpunit polyfills dependency

Author: tschortsch

composer.json

@@ -27,7 +27,8 @@
 		"squizlabs/php_codesniffer": "^3.6",
 		"wp-coding-standards/wpcs": "^2.3",
 		"dealerdirect/phpcodesniffer-composer-installer": "^0.7.1",
-		"wp-phpunit/wp-phpunit": "^5.8"
+		"wp-phpunit/wp-phpunit": "^5.8",
+		"yoast/phpunit-polyfills": "^1.0"
 	},
 	"scripts": {
 		"lint": "phpcs --standard=phpcs.xml"