Add test for pre closed and closed assumptions

[holzeis]

These are fair assumptions, given that the PreClosed and Closed variants are supposed to refer to a force-closed channel if the attestations field is set.

But looking at the rust-dlc code, I'm pretty sure this won't work. AFAICT, if we find a confirmed CET we are setting it to PreClosed (?!) and we are setting attestations to None.

It would be good to test this PR e2e, but for that we need #2378.

Originally posted by @luckysori in https://github.com/get10101/10101/pull/2513#discussion_r1594875953