[flake]: TestBatchedAssetIssuance flake

[jtobin]

Our old nemesis, tapgarden (tapgarrrrrdddddennnnnnnnnnn!), has surfaced yet another flake:

--- FAIL: TestBatchedAssetIssuance (113.21s)
    --- FAIL: TestBatchedAssetIssuance/basic_asset_creation (66.74s)
        test_sqlite.go:11: Creating new SQLite DB handle for testing: /tmp/TestBatchedAssetIssuancebasic_asset_creation2778551181/001/tmp.db
        planter_test.go:1186: 
            	Error Trace:	/home/runner/work/taproot-assets/taproot-assets/tapgarden/planter_test.go:714
            	            				/home/runner/work/taproot-assets/taproot-assets/tapgarden/planter_test.go:1186
            	            				/home/runner/work/taproot-assets/taproot-assets/tapgarden/planter_test.go:2104
            	Error:      	Received unexpected error:
            	            	predicate not satisfied after time out
            	Test:       	TestBatchedAssetIssuance/basic_asset_creation

.. which appeared in this unit-race job.

[jtobin]

A different flake, but also for TestBatchedAssetIssuance, occurred here:

--- FAIL: TestBatchedAssetIssuance (6.70s)
    --- FAIL: TestBatchedAssetIssuance/creation_by_minting_ticker (3.39s)
        test_sqlite.go:11: Creating new SQLite DB handle for testing: /tmp/TestBatchedAssetIssuancecreation_by_minting_ticker3832361883/001/tmp.db
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1a78d5b]