Add unit test for last hop stopped #2044

Job	Run time
e2e test for 3-nodes-transfer --env test	4m 34s
e2e test for invoice-ops --env test	3m 57s
e2e test for open-use-close-a-channel --env test	4m 6s
e2e test for udt --env test	4m 24s
e2e test for reestablish --env test	4m 19s
e2e test for cross-chain-hub --env test	4m 18s
e2e test for router-pay --env test	5m 33s
e2e test for udt-router-pay --env test	4m 20s
e2e test for watchtower --env test	5m 39s
e2e test for udt --env xudt-test	4m 32s
e2e test for 3-nodes-transfer --env test --env-var HASH_ALGORITHM=sha256	4m 47s
	50m 29s

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