Fix flaky relay-htlc-add test (#1752)

We need to wait for the channel relayer to be registered to the event stream
before publishing an event, otherwise the channel relayer will never receive it.

We send it a first message and wait for its response to ensure it had time
to correctly initialize.

Author: t-bast

eclair-core/src/test/scala/fr/acinq/eclair/payment/relay/RelayerSpec.scala

@@ -71,9 +71,13 @@ class RelayerSpec extends ScalaTestWithActorTestKit(ConfigFactory.load("applicat
   test("relay an htlc-add") { f =>
     import f._
 
-    system.eventStream ! EventStream.Publish(LocalChannelUpdate(null, channelId_bc, channelUpdate_bc.shortChannelId, c, None, channelUpdate_bc, makeCommitments(channelId_bc)))
-
+    // We make sure the channel relayer is initialized
     val sender = TestProbe[Relayer.OutgoingChannels]()
+    childActors.channelRelayer ! ChannelRelayer.GetOutgoingChannels(sender.ref.toClassic, GetOutgoingChannels())
+    assert(sender.expectMessageType[Relayer.OutgoingChannels].channels.isEmpty)
+
+    // We publish a channel update, that should be picked up by the channel relayer
+    system.eventStream ! EventStream.Publish(LocalChannelUpdate(null, channelId_bc, channelUpdate_bc.shortChannelId, c, None, channelUpdate_bc, makeCommitments(channelId_bc)))
     eventually(PatienceConfiguration.Timeout(30 seconds), PatienceConfiguration.Interval(1 second)) {
       childActors.channelRelayer ! ChannelRelayer.GetOutgoingChannels(sender.ref.toClassic, GetOutgoingChannels())
       val channels = sender.expectMessageType[Relayer.OutgoingChannels].channels