AddStringAttachmentTest: add additional "fail to attach" test method
AddStringAttachmentTest: add additional "fail to attach" test method

This commit:
* Adds a new `testFailToAttach()` test method to test the behaviour of the `PHPMailer::addStringAttachment()` method when the `PHPMailer` class has been instantiated with `$exceptions` disabled.
* This new test method uses the same data provider - introduced in the previous commit - as the `testFailToAttachException()` method.

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