SetErrorTest: replace the original test
SetErrorTest: replace the original test

... by a much simpler test which effectively tests the same thing, i.e.:
* No errors to start with.
* Trigger an error.
* Verify that `PHPMailer::isError()` returns `true`.
* Verify that the error message is as expected.

jimjag/PHPMailer,PHPMailer/PHPMailer,PHPMailer/PHPMailer,nawawi/PHPMailer,nawawi/PHPMailer,jimjag/PHPMailer