tapgarden: reduce flakiness in TestBatchedAssetIssuance

[ffranr]

Assign a separate minting store to each test case to avoid shared state and improve test reliability.

I think I've noticed this unit test flake a few times in CI.

[coveralls]

Pull Request Test Coverage Report for Build 17301563467

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 27 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+8.0%) to 56.683%

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Files with Coverage Reduction	New Missed Lines	%
fn/context_guard.go	1	91.94%
tapdb/mssmt.go	2	89.55%
tapgarden/custodian.go	2	77.02%
itest/multisig.go	3	97.49%
itest/assertions.go	6	88.93%
mssmt/compacted_tree.go	6	77.97%
tapdb/assets_store.go	7	79.29%

Totals
Change from base Build 17301129198:	8.0%
Covered Lines:	61025
Relevant Lines:	107661

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💛 - Coveralls

[jtobin]

Makes sense, LGTM.

[GeorgeTsagk]

Lgtm!