What's the best way to test Digest Emails in Migration/UAT environment?

Not specifically, you would have to create the content and trigger the digest as you would in a production environment

How do you "trigger the digest"?  Is there a way to have that happen immediately?

It looks like you can change the time and the frequency under Administration > Jobs > Email Digest Daily, but maybe there is a better way.

You can change the job frequency which will make the digest send more quickly IF it is time to send it - however, each subscription still only sends at the intended frequency (daily, weekly), controlled by the Email Digest Subscription's NextSendDate.