channels: fix coop close asset distribution for grouped asset channels

[guggero]

What this issue fixes is a bit hard to explain. So I'll just copy the comment that's above the fix:

	// [...] When we [coop] close a channel, the output of the
	// `lncli closedchannels` command will show the last commitment state of
	// the channel as the closing asset balance. Which is correct in terms
	// of balances. But if there are multiple different asset IDs (e.g., in
	// a grouped asset channel), then _how_ those pieces are distributed
	// within the commitment transaction depends on the order of the
	// allocations. And the order of the allocations is dependent on the
	// BTC amount and the pkScript of the BTC-level output. Both of which
	// are different in the coop close output (we set the asset-level output
	// BTC amount to the dummy amount, and the pkScript will be a newly
	// derived internal key with no sibling script path).
	// So, long story short: If we used the tapsend.DistributeCoins method
	// here, it could happen that the actual asset output distribution shown
	// in the `lncli closedchannels` command would be different from the
	// actual distribution in the co-op close transaction.
	// This could mostly be seen as an UX-only issue, but was actually
	// discovered while attempting to assert the final closing balance of
	// grouped asset channels in the litd integration test.

[guggero]

Corresponding itest case that found this issue is here: lightninglabs/lightning-terminal#1056

[coveralls]

Pull Request Test Coverage Report for Build 14883073599

Details

• 18 of 121 (14.88%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on coop-close-allocation-sort at 36.907%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tapchannel/aux_closer.go	0	13	0.0%
tapchannel/commitment.go	0	90	0.0%

Totals
Change from base Build 14866822237:	36.9%
Covered Lines:	26474
Relevant Lines:	71732

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💛 - Coveralls

[Roasbeef]

LGTM 📩

[ffranr]

Thanks for the added context in 1:1