ReplyToGetSetClearTest: improve the conversion and enqueuing test
ReplyToGetSetClearTest: improve the conversion and enqueuing test

This commit:
* Renames the `testConvertEncoding()` method to `testEnqueueAndAddIdnAddress()`.
* Removes the call to `clearReplyTos()` at the start of the function as it is redundant. Each test gets a fresh instance of the PHPMailer class.
* Adds a number of additional assertions to the method to test the enqueuing in more depth.

Includes adding `@covers` tags for the test.

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