Crashes core's Save and Send Test Email function

[artfulrobot]

Save & Send causes a really good crash on a white screen:

Call to undefined method class@anonymous::getDriver() in CRM_Utils_Mail::sendTest()
   (line 253 of /var/www/example.com/web/vendor/civicrm/civicrm-core/CRM/Utils/Mail.php).

There's also deprecated messages too re assigning to $this->mailer.

This is because Core assumes it can use $mailer->getDriver() but because $mailer is replaced by this extension, it doesn't have it.

Core could be fixed to be more hardy against extensions that replace $mailer with something else, but I think the fix should be in here because we're breaking a fairly sensible contract requirement of core.

Nb. affects 5.78.3 not sure what else. It's possible that pr 5151 broke the extensino code.

[bjendres]

possible related to #47?