AddStringEmbeddedImageTest: improve original test [1]
AddStringEmbeddedImageTest: improve original test [1]

This commit:
* Improves the test name and the description in the docblock.
* Replace a redundant condition and "forced" failure assertion with an assertion actually testing the result of the method call.
* Removes the redundant `return` - if an assertion fails, the rest of the code within the test method will not be executed anyway.
* Minor inline comment tweaks.

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