Specify phpunit schema

Author: zbateson

tests/phpunit.xml

@@ -1,13 +1,15 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <phpunit
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     bootstrap="./bootstrap.php"
     colors="true"
     convertNoticesToExceptions="true"
     convertWarningsToExceptions="true"
     stopOnError="false"
     stopOnFailure="false"
     stopOnIncomplete="false"
-    stopOnSkipped="false">
+    stopOnSkipped="false"
+    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd">
     <coverage processUncoveredFiles="true">
         <include>
             <directory suffix=".php">../src/</directory>