Provide guidance and/or sane defaults for `Testnet` fee rates

[tnull]

Users keep running into fee rate related interoperability issues on Testnet as we try to dynamically estimate fees while CLN/LND hard-code certain fee rates due to fee estimation being broken on Testnet.

We should fix this, either by also hardcoding compatible values for testnet, or providing clear guidance in the FeeEstimator docs which values the user should set in their implementation.

[vincenzopalazzo]

I noted this too in lampo, but nowadays getting some funds on testnet looks hard due to block storms.

IIRC fee disagreement causes force closes in LDK, right? there is a possibility to set in the configuration a way to "disable the channel" and not close it?

I also remember a talk with @TheBlueMatt a while ago that disabling the channel was a vulnerability (but I could be wrong)

[TheBlueMatt]

Indeed, pre-Bitcoin Core 28, fee rate disagreements mean the channel is in a state where you cannot force-close it. We either have to accept that or force-close immediately. Luckily, with anchor channels and Bitcoin Core 28, ignoring pinning attacks, we can finally accept any feerate!