`background-processor` tests are flaky

[TheBlueMatt]

I think we regressed with #2141 but it seems like a once-in-1000 crash so need to debug further.

[TheBlueMatt]

Actually no idea if this regressed or not. Not sure how we can hit this but certainly can't be async related.

[TheBlueMatt]

Removing milestone since it isn't clear to me its a regression and I confused it for another issue I had locally, hopefully the debugging in #2145 makes it clearer whats going on here.

[TheBlueMatt]

Hit this again on macos, 2145 didn't turn up any further debug info :( https://github.com/lightningdevkit/rust-lightning/actions/runs/4630693695/jobs/8192606771?pr=2023

[TheBlueMatt]

Failed again on macOS at #2163 I'm starting to think its a mac issue...