routing: run path finding tests against cache and DB

To make sure we don't hit any edge cases we refactor the path finding
tests to run both with and without the cache enabled.

routing/payment_lifecycle_test.go

@@ -180,7 +180,7 @@ func TestRouterPaymentStateMachine(t *testing.T) {
 		}, 2),
 	}
 
-	testGraph, err := createTestGraphFromChannels(testChannels, "a")
+	testGraph, err := createTestGraphFromChannels(true, testChannels, "a")
 	if err != nil {
 		t.Fatalf("unable to create graph: %v", err)
 	}