Make a test plan and ensure it's only accessible to super users for example then test it. I recommend a free plan or merchandise plan as that'll best let you test quickly.

OK - it is a bit long winded....

Not sure what you were expecting. We've no method for any emails to directly test them without performing the action they're dependent on. If you just want to do a simple HTML format test you can use the CB > User Management > Mass Mailer to send an email to your self.

BTW, in your forum post notification emails, your url is missing from the link - you only get 'http://forum/255-developer-members-support/234259-cbssubs-test-plan-emails#284045'

Yup, known Kunena bug.

OK, what about a HTML editor for these messages?

WYSIWYG editors are a bit rude in Joomla. They like to take what you give them and alter the heck out of them to fit whatever ideal of good code/format they have in mind. This breaks more than it helps and causes headache. So we choose to use them extremely sparingly. I highly recommend just writing proper valid HTML in an IDE then copy it over (e.g. PHPStorm, Netbeans, Notepad++, etc...). Even better is to save it as an HTML file and you can test what your HTML looks like without ever sending an email.

BTW, can I put HMTL (e.g. design something in Acymail then cut/paste code) into the message box?

Yes.