Add CountDownLatch to ChannelMultiplexer test

ChannelMultiplexer is very dependent on subscribing order, and should be
either set up correctly or used by subscribers that don't really care
about past events.

shared/src/test/scala/ChannelBehavior.scala

@@ -356,10 +356,12 @@ class ChannelBehavior extends munit.FunSuite {
   test("ChannelMultiplexer multiple readers and writers") {
     Async.blocking:
       val m = ChannelMultiplexer[Int]()
+      val start = java.util.concurrent.CountDownLatch(5)
 
       val f11 = Future:
         val cc = SyncChannel[Int]()
         m.addPublisher(cc)
+        start.countDown()
         sleep(200)
         for (i <- 0 to 3)
           cc.send(i)
@@ -368,6 +370,7 @@ class ChannelBehavior extends munit.FunSuite {
       val f12 = Future:
         val cc = SyncChannel[Int]()
         m.addPublisher(cc)
+        start.countDown()
         sleep(200)
         for (i <- 10 to 13)
           cc.send(i)
@@ -376,13 +379,15 @@ class ChannelBehavior extends munit.FunSuite {
       val f13 = Future:
         val cc = SyncChannel[Int]()
         m.addPublisher(cc)
+        start.countDown()
         for (i <- 20 to 23)
           cc.send(i)
         m.removePublisher(cc)
 
       val f21 = Future:
         val cr = SyncChannel[Try[Int]]()
         m.addSubscriber(cr)
+        start.countDown()
         sleep(200)
         val l = ArrayBuffer[Int]()
         sleep(1000)
@@ -393,12 +398,14 @@ class ChannelBehavior extends munit.FunSuite {
       val f22 = Future:
         val cr = SyncChannel[Try[Int]]()
         m.addSubscriber(cr)
+        start.countDown()
         sleep(1500)
         val l = ArrayBuffer[Int]()
         for (i <- 1 to 12) {
           l += cr.read().right.get.get
         }
 
+      start.await()
       Future { m.run() }
 
       f21.result