AddEmbeddedImageTest: split off failure test
AddEmbeddedImageTest: split off failure test

The "failure" case when a non-existent file was being passed, wasn't actually being tested at all as no assertion was used.

This commit:
* Moves the particular failure test case to a separate test method with a data provider (to allow for more failure test cases to be added).
* Uses an assertion on the call to `addEmbeddedImage()` to actually test that the method return a failure state.
* Verifies that no attachment for an inline image was added by adding a second assertion with a call to `PHPMailer::inlineImageExists()`.

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