routing/test: remove test channel buffers

Now that we run each test individually, we don't need to buffer
our mock's channels anymore. This helps to tighten our test loop,
which currently can move on from a step before it's actually
been processed by the mock. This removal ensures that our payment
loop processes each of the test's steps before moving on to the
next once.

routing/payment_lifecycle_test.go

@@ -598,12 +598,12 @@ func testPaymentLifecycle(t *testing.T, test paymentLifecycleTestCase,
 	// Create a mock control tower with channels set up, that we use to
 	// synchronize and listen for events.
 	control := makeMockControlTower()
-	control.init = make(chan initArgs, 20)
-	control.registerAttempt = make(chan registerAttemptArgs, 20)
-	control.settleAttempt = make(chan settleAttemptArgs, 20)
-	control.failAttempt = make(chan failAttemptArgs, 20)
-	control.failPayment = make(chan failPaymentArgs, 20)
-	control.fetchInFlight = make(chan struct{}, 20)
+	control.init = make(chan initArgs)
+	control.registerAttempt = make(chan registerAttemptArgs)
+	control.settleAttempt = make(chan settleAttemptArgs)
+	control.failAttempt = make(chan failAttemptArgs)
+	control.failPayment = make(chan failPaymentArgs)
+	control.fetchInFlight = make(chan struct{})
 
 	// setupRouter is a helper method that creates and starts the router in
 	// the desired configuration for this test.