Enhancement: Reference phpunit schema as installed with composer

Author: localheinz

phpunit.xml

@@ -1,4 +1,8 @@
-<phpunit bootstrap="vendor/autoload.php">
+<phpunit
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+    bootstrap="vendor/autoload.php"
+>
     <testsuites>
         <testsuite>
             <directory>tests</directory>
@@ -9,4 +13,4 @@
             <directory suffix=".php">src</directory>
         </whitelist>
     </filter>
-</phpunit>
+</phpunit>