[10.3] Add void return typehints to all test methods

See core issue: #3421418: [May 2024] Add void return typehints to all test methods

This was already done in Mime Mail more than a year ago.
However, one of our tests subclasses from the core PhpMail test, and when the core PhpMail test was changed in the above issue, we now have a different signature that needs to be fixed.