With the new, API-controlled accounts, you'll be able to create ample test users without violating the TOS, and Facebook won't disable the profile for being fake or for being a spam account.
Devs can create one test user, or an application can make up to 50 test accounts. Any test account can be accessed by any developer associated with that application. To create a test user account, just use the Graph API with the application access token.
With the API, you can make friend connections between various test user accounts without having to log in as one particular user and manually accept friend requests. This nicety should make testing app actions among groups of friends a lot easier.
Once you're done with the test accounts, they can be easily deleted.
Also, as Facebook developer relations head Douglas Purdy noted in the comments on Facebook's blog post on the subject, test users will be helpful to Facebook engineers working on platform stability.
"This feature is absolutely key for us to be able to work on stability and bugs," he wrote. "Our hope is that this lets folks build automated tests to give us an early warning into any breaking changes we miss... This should help people give us narrowed repro of bugs they encounter in their test suites."
There are a few limits as to what you can do with a test user. Test accounts can't interact with real users, and they can't interact with public content, such as a post on a Page. Test users will only have test privileges for the app, and they can't be converted to non-test accounts.
Given these improvements in how Facebook handles app testing, the company plans to disable all old test accounts and its developer test network by the end of the year.