routing/test: use half shard for single success case

Update our single shard success case to use a route which
splits the payment amount in half. This change still tests
the case where reveal of the preimage counts as a success,
even if we don't have the full amount. This change is made
to cut down on potential races in this test case. While we
are waiting for collectResultAsync to report a success, the
payment lifecycle will continue trying to dispatch shards.
In the case where we send 1/4 of the payment amount, we
send 1 or 2 more shards, depending on how long collectAsync
takes. Reducing this test to send 1/2 of the payment amount
means that we will always only try one more shard before
waiting for our shard.

routing/payment_lifecycle_test.go

@@ -190,6 +190,9 @@ func TestRouterPaymentStateMachine(t *testing.T) {
 		t.Fatalf("unable to create route: %v", err)
 	}
 
+	halfShard, err := createTestRoute(paymentAmt/2, testGraph.aliasMap)
+	require.NoError(t, err, "unable to create half route")
+
 	shard, err := createTestRoute(paymentAmt/4, testGraph.aliasMap)
 	if err != nil {
 		t.Fatalf("unable to create route: %v", err)
@@ -501,10 +504,8 @@ func TestRouterPaymentStateMachine(t *testing.T) {
 			},
 		},
 		{
-			// An MP payment scenario where 3 of the shards fail.
+			// An MP payment scenario where one of the shards fails,
-			// However the last shard settle, which means we get
+			// but we still receive a single success shard.
-			// the preimage and should consider the overall payment
-			// a success.
 			name: "MP one shard success",
 
 			steps: []string{
@@ -518,30 +519,18 @@ func TestRouterPaymentStateMachine(t *testing.T) {
 				routerRegisterAttempt,
 				sendToSwitchSuccess,
 
-				// shard 2
+				// shard 0 fails, and should be failed by the
-				routerRegisterAttempt,
-				sendToSwitchSuccess,
-
-				// shard 3
-				routerRegisterAttempt,
-				sendToSwitchSuccess,
-
-				// 3 shards fail, and should be failed by the
 				// router.
 				getPaymentResultTempFailure,
-				getPaymentResultTempFailure,
-				getPaymentResultTempFailure,
-				routerFailAttempt,
-				routerFailAttempt,
 				routerFailAttempt,
 
-				// The fourth shard succeed against all odds,
+				// The second shard succeed against all odds,
 				// making the overall payment succeed.
 				getPaymentResultSuccess,
 				routerSettleAttempt,
 				paymentSuccess,
 			},
-			routes: []*route.Route{shard, shard, shard, shard},
+			routes: []*route.Route{halfShard, halfShard},
 		},
 		{
 			// An MP payment scenario a shard fail with a terminal