Add E2E test for background functions

[Skn0tt]

In #2058 we introduced a regression around experimental-background functions. It was fixed in https://github.com/netlify/next-runtime/pull/2121, and could've been caught by an E2E test.

We should build an E2E test that asserts on background functions running to completion, maybe using a tool like https://webhook.site/.

[nickytonline]

We'll be taking this over from @Skn0tt.

[MarcL]

Closing this as we won't be doing this in the v4 runtime now we have solid testing in the v5 runtime.