Flaky fixes

[rustyrussell]

This is based on #8767 (which was causing one of the "flakes"!).

This is a draft because the final commit disables all flakes so I can catch them (I'm also looping it on my laptop), so I can root cause the others. We may still mark some flaky if they are genuinely unreliable, but we need to make that determination!

Changelog-None