Say it's broken. Say you're thinking about something else entirely and don't have a lot of patience for this bug. Which test do you wish had been written now?
I test as much as I can get away with. At some point you have to stop and get paid.
Keeping that in mind, it's not simply behavor. It's the boundaries. Your behavor is 'show me the neighbors'. Your boundarys where the logic of that behavor changes are at 0,1,7, 8, 9, 16, 57, 58, & 63. These generate every shape posible around your square.
That's 9 tests I personally would rather see done in your first style before breaking them down for your second style. It's better to catch more and report it sloppy than to report exactly and miss much.
Ideally, If I was debugging your code (and some sayday I might be) I'd want all 9 in the second style. But in 20 years of coding I've yet to see 'ideal' code.
Please give me as much as you can.