Add xml schema to phpunit (#5139)

This allows an IDE to do auto completion, and show any errors in the configuration

phpunit.xml

@@ -1,5 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit backupGlobals="false"
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="./vendor/phpunit/phpunit/phpunit.xsd"
+         backupGlobals="false"
          backupStaticAttributes="false"
          bootstrap="vendor/autoload.php"
          colors="true"