Add tests for warning messages from peers

[positiveblue]

Change Description

Test that warning messages from our peers do not trigger any action.

Steps to Test

pushd htlcswitch && go test -v -run TestChannelLinkFail && popd

Pull Request Checklist

Testing

• [x] Your PR passes all CI checks.
• [x] Tests covering the positive and negative (error paths) are included.

Code Style and Documentation

• [x] The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• [x] Commits follow the Ideal Git Commit Structure.
• [x] There is a change description in the release notes, or [skip ci] in the commit message for small changes.

[Roasbeef]

This test confirms everything in the link was ok, but the root issue was that we converted the warning to an error when sending to the link in the first place: https://github.com/lightningnetwork/lnd/issues/6801#issuecomment-1208254268

[lightninglabs-deploy]

@crypt-iq: review reminder @positiveblue, remember to re-request review from reviewers when ready

[Crypt-iQ]

needs rebase

[positiveblue]

Close in favor of #6840