Hi All. I figured it out based on a related thread (conveniently displayed to the right of my post): https://nation.marketo.com/message/115192#comment-115192
Turns out that the cookie is not set with TEST emails but it does work with REGULAR emails. So, just had to re-run my test with a regular email to an audience of 1 (me) and it works. I see my activity in the Activity Log (retroactive to when I last cleared my cookies) and now forms pre-populate with my contact info - even though I've never filled out a form with this contact info. A simple email sent to a known user in Marketo did the trick!