Use MAIL_MAILER in test environment (#5239)

phpunit.xml

@@ -23,7 +23,7 @@
         <server name="CACHE_DRIVER" value="array"/>
         <server name="DB_CONNECTION" value="sqlite"/>
         <server name="DB_DATABASE" value=":memory:"/>
-        <server name="MAIL_DRIVER" value="array"/>
+        <server name="MAIL_MAILER" value="array"/>
         <server name="QUEUE_CONNECTION" value="sync"/>
         <server name="SESSION_DRIVER" value="array"/>
     </php>