client: fill LastHop in FetchSwaps

Loop used to "forget" last_hop of swaps created before its restart.
The commit fixes this.

Added test TestFetchSwapsLastHop for this. Filled field abandonChans in test
utility function newSwapClient because the field is used in new test.

Fixes https://github.com/lightninglabs/loop/issues/798

testcontext_test.go

@@ -108,6 +108,7 @@ func newSwapClient(t *testing.T, config *clientConfig) *Client {
 		sweeper:      sweeper,
 		executor:     executor,
 		resumeReady:  make(chan struct{}),
+		abandonChans: make(map[lntypes.Hash]chan struct{}),
 	}
 }