Overhaul our tests because our current scaffolding doesn't work with many of the new app-dir tests

[nickytonline]

From @orinokai:

we need to overhaul our tests because our current scaffolding doesn't work with many of the new app-dir tests - i think it's important to get a basic level of support shipped and then work on getting these tests in, but shout if you disagree

Originally posted by @orinokai in https://github.com/netlify/next-runtime/pull/2080#discussion_r1228460598