PHPUnit config: add schema references (sirbrillig#245)

Co-authored-by: jrfnl <jrfnl@users.noreply.github.com>

Author: jrfnl

phpunit.xml.dist

@@ -1,4 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
 <phpunit
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/9.0/phpunit.xsd"
 	bootstrap="Tests/bootstrap.php"
 	convertErrorsToExceptions="true"
 	convertWarningsToExceptions="true"