multi: increase commitment v0/v1/v2 unit test coverage

[guggero]

Adds more unit tests around the commitment v0/v1/v2 formats.

[coveralls]

Pull Request Test Coverage Report for Build 17121949593

Details

• 13 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• 85 unchanged lines in 18 files lost coverage.
• Overall coverage decreased (-0.02%) to 56.641%

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Files with Coverage Reduction	New Missed Lines	%
authmailbox/receive_subscription.go	1	79.25%
address/mock.go	2	95.11%
commitment/tap.go	2	85.19%
tapdb/sqlc/transfers.sql.go	2	82.65%
tapdb/universe.go	2	81.64%
universe/syncer.go	2	82.73%
asset/asset.go	3	80.15%
asset/mock.go	3	73.21%
itest/multisig.go	3	97.94%
tapchannel/aux_leaf_signer.go	3	43.43%

Totals
Change from base Build 17057360543:	-0.02%
Covered Lines:	60687
Relevant Lines:	107143

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💛 - Coveralls

[Roasbeef]

LGTM 🥓