[flake]: suspected custom_channels_htlc_force_close_MPP flake

[jtobin]

custom_channels_htlc_force_close_MPP was found failing in CI on #1775. I haven't seen this before, but the changes in that PR certainly shouldn't trigger it. The error, from this job:

    assets_test.go:3608: 
        	Error Trace:	/home/runner/work/taproot-assets/taproot-assets/lightning-terminal/itest/assets_test.go:3608
        	            				/home/runner/work/taproot-assets/taproot-assets/lightning-terminal/itest/litd_custom_channels_test.go:4680
        	            				/home/runner/work/taproot-assets/taproot-assets/lightning-terminal/itest/litd_custom_channels_test.go:4509
        	            				/home/runner/work/taproot-assets/taproot-assets/lightning-terminal/itest/test_harness.go:103
        	            				/home/runner/work/taproot-assets/taproot-assets/lightning-terminal/itest/litd_test.go:138
        	Error:      	Received unexpected error:
        	            	wanted 2, found 1 txs in mempool: [07c4b7ac79d7031f3507ad86839b7225e5f2f7a9f04586b14b108ed79b50446e]
        	Test:       	TestLightningTerminal/tranche00/17-of-29/custom_channels_htlc_force_close_MPP

I feel this is likely a concurrency issue.