musig2: fix early nonce gen option

[sputn1ck]

Previously the early nonce generation option was not being respected when creating the context, with the WithKnownSigners option being used.

[coveralls]

Pull Request Test Coverage Report for Build 5519660551

• 7 of 9 (77.78%) changed or added relevant lines in 1 file are covered.
• 10 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 55.24%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
btcec/schnorr/musig2/context.go	7	9	77.78%
<!--	Total:	7	9	77.78%	-->

Files with Coverage Reduction	New Missed Lines	%
peer/peer.go	10	73.2%
<!--	Total:	10		-->

Totals
Change from base Build 5512933726:	-0.02%
Covered Lines:	26692
Relevant Lines:	48320

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💛 - Coveralls

[sputn1ck]

Motivation makes sense, can we tack on a unit test to ensure the new code path works as expected and as a guard against regressions?

I adapted the early nonce gen test to use both ways of creating a context. Can also add a completely new unit test if preferred.