set phpunit version in config

Author: jrushlow

phpunit.xml.dist

@@ -16,6 +16,7 @@
         <ini name="error_reporting" value="-1" />
         <env name="TEST_DATABASE_DSN" value="mysql://root:[email protected]:3306/test_maker?serverVersion=5.7" />
         <env name="SYMFONY_DEPRECATIONS_HELPER" value="max[self]=0"/>
+        <env name="SYMFONY_PHPUNIT_VERSION" value="9.6" />
         <env name="MAKER_SKIP_MERCURE_TEST" value="false"/>
     </php>