docs: restore phpunit.xml

Author: kenjis

tests/README.md

@@ -37,7 +37,7 @@ A number of the tests use a running database.
 The default configuration uses SQLite3 memory database, so it works if you can use SQLite3.
 
 In order to change the database for testing, edit the details for the `tests` group in
-**app/Config/Database.php** or use **.env** file.
+**app/Config/Database.php** or **phpunit.xml** or use **.env** file.
 
 E.g.:
 ```