RE: Is there a way to send a plain text version of the email as a test email?
Hi Adele,
Welcome to join the Dynamics 365 Community!
As per my understanding, you can insert dynamic expression for "subscription center" or "view online" into marketing email as text content directly instead of having to attach them to text.
Preview in Litmus
In addition to Litmus, you could also try "Test send".
https://docs.microsoft.com/en-us/dynamics365/marketing/email-preview
However, the feature has some limitations,(such as the subscription center won't function.) which have been mentioned in the documentation.
If you would like to test full functionality of marketing email, please create a segment and a simple customer journey to test.
Regards,
Clofly