itest: add breach itest for custom channels

[Roasbeef]

In this commit, we add a breach itest for the custom channels. The scenario is as follows:

• Two nodes: Charlie and Dave.
• Charlie does asset keysends to Dave.
• Dave snapshots his database.
• Charlie sends another keysend.
• Dave breaches.
• Charlie sweeps both asset UTXOs into a new anchor output after the breach confirms.

[guggero]

Very cool 😎
LGTM 🎉

[guggero]

CI failures look like a flake, couldn't reproduce locally. Re-running failed steps.

[GeorgeTsagk]

🟢

[dstadulis]

This will be merged once both go module commit references of the lnd/tapd merged PRs are updated