lntest: use nextAvailablePort for fee service

lntest/fee_service.go

@@ -16,9 +16,6 @@ const (
 	// is returned. Requests for higher confirmation targets will fall back
 	// to this.
 	feeServiceTarget = 2
-
-	// feeServicePort is the tcp port on which the service runs.
-	feeServicePort = 16534
 )
 
 // feeService runs a web service that provides fee estimation information.
@@ -40,16 +37,15 @@ type feeEstimates struct {
 
 // startFeeService spins up a go-routine to serve fee estimates.
 func startFeeService() *feeService {
+	port := nextAvailablePort()
 	f := feeService{
-		url: fmt.Sprintf(
+		url: fmt.Sprintf("http://localhost:%v/fee-estimates.json", port),
-			"http://localhost:%v/fee-estimates.json", feeServicePort,
-		),
 	}
 
 	// Initialize default fee estimate.
 	f.Fees = map[uint32]uint32{feeServiceTarget: 50000}
 
-	listenAddr := fmt.Sprintf(":%v", feeServicePort)
+	listenAddr := fmt.Sprintf(":%v", port)
 	f.srv = &http.Server{
 		Addr: listenAddr,
 	}