Proxys

I would find a manager that has employees, hard code that manager in whatever query needs it, then manually kick off the flow in the Process Triggers screen

Tim's approach would work.

Although proxy is a possibility, it adds another level of complexity which could impact your testing. Another option would be to have your security folks create a test user (in DEV/QA or whatever) that is linked to a manager record and then test using that user (or better, those users)? This is very common in our environment. We "check out" test users from our security team so they know who is doing what in the system.

If they have concerns about you seeing "real" employee data, they could create a number of "fake" employees in DEV and assign them to a "fake" manager, and then create a test user attached to that fake manager for testing. It's a little cumbersome, but it's really not that big of a deal if done in DEV.