Tests: introduce dedicated tests for the `PHPMailer::html2text()` method
Tests: introduce dedicated tests for the `PHPMailer::html2text()` method

So far, this method did not have dedicated tests.

The test file this commit introduces, tests all aspects of the method as well as documents the current behaviour of the method for specific, outlier situations.

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