Use `<env>` tags for configuration

`<env>` tags have a lower precedence than system environment variables making it easier to overwrite PHPUnit configuration values in additional environments, such a CI.

Review this blog post for more details on configuration precedence when testing Laravel: https://jasonmccreary.me/articles/laravel-testing-configuration-precedence/

phpunit.xml

@@ -18,15 +18,15 @@
         </include>
     </coverage>
     <php>
-        <server name="APP_ENV" value="testing"/>
-        <server name="BCRYPT_ROUNDS" value="4"/>
-        <server name="CACHE_DRIVER" value="array"/>
-        <server name="DB_CONNECTION" value="mysql"/>
-        <server name="DB_DATABASE" value="controlpanel_test"/>
-        <server name="MAIL_MAILER" value="array"/>
-        <server name="QUEUE_CONNECTION" value="sync"/>
-        <server name="SESSION_DRIVER" value="array"/>
-        <server name="TELESCOPE_ENABLED" value="false"/>
+        <env name="APP_ENV" value="testing"/>
+        <env name="BCRYPT_ROUNDS" value="4"/>
+        <env name="CACHE_DRIVER" value="array"/>
+        <env name="DB_CONNECTION" value="mysql"/>
+        <env name="DB_DATABASE" value="controlpanel_test"/>
+        <env name="MAIL_MAILER" value="array"/>
+        <env name="QUEUE_CONNECTION" value="sync"/>
+        <env name="SESSION_DRIVER" value="array"/>
+        <env name="TELESCOPE_ENABLED" value="false"/>
         <ini name="display_errors" value="true"/>
     </php>
 </phpunit>