[itest flake]: custom channels `test_custom_channels_htlc_force_close` flake

[ellemouton]

The following 2 flakes in the test_custom_channels_htlc_force_close test occur quite frequently in our CI:

Flake 1

    litd_custom_channels_test.go:3204: 
        	Error Trace:	/home/runner/work/lightning-terminal/lightning-terminal/itest/litd_custom_channels_test.go:3204
        	            				/home/runner/work/lightning-terminal/lightning-terminal/itest/litd_custom_channels_test.go:2924
        	            				/home/runner/work/lightning-terminal/lightning-terminal/itest/test_harness.go:103
        	            				/home/runner/work/lightning-terminal/lightning-terminal/itest/litd_test.go:102
        	Error:      	Received unexpected error:
        	            	wanted 1, found 0 txs in mempool: []
        	Test:       	TestLightningTerminal/test_custom_channels_htlc_force_close

Flake 2

    assets_test.go:350: 
        	Error Trace:	/home/runner/work/lightning-terminal/lightning-terminal/itest/assets_test.go:350
        	            				/home/runner/work/lightning-terminal/lightning-terminal/itest/litd_custom_channels_test.go:3311
        	            				/home/runner/work/lightning-terminal/lightning-terminal/itest/litd_custom_channels_test.go:2923
        	            				/home/runner/work/lightning-terminal/lightning-terminal/itest/test_harness.go:103
        	            				/home/runner/work/lightning-terminal/lightning-terminal/itest/litd_test.go:102
        	Error:      	Received unexpected error:
        	            	missing anchor block hash, transfer not confirmed
        	Test:       	TestLightningTerminal/test_custom_channels_htlc_force_close