add multi-sig test for AtLeast 2/3, 3/4 and 7/10;

[greenhat]

Trying to reproduce #692

[coveralls]

Pull Request Test Coverage Report for Build 4402416642

• 18 of 20 (90.0%) changed or added relevant lines in 3 files are covered.
• 20 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.07%) to 82.925%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergo-lib/src/wallet.rs	0	1	0.0%
ergo-lib/src/wallet/multi_sig.rs	8	9	88.89%

Files with Coverage Reduction	New Missed Lines	%
ergo-merkle-tree/src/merkletree.rs	2	97.39%
ergo-merkle-tree/src/batchmerkleproof.rs	3	90.48%
ergotree-ir/src/chain/token.rs	5	72.58%
ergotree-ir/src/serialization/types.rs	10	80.77%

Totals
Change from base Build 4402412703:	-0.07%
Covered Lines:	9927
Relevant Lines:	11971

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💛 - Coveralls