UntypedMqttFlowSpec: mqtt client flow should establish a bidirectional connection and subscribe to a topic

Possibly related to #468

```
[info] UntypedMqttFlowSpec:
[info] mqtt client flow
[info] - should establish a bidirectional connection and subscribe to a topic *** FAILED *** (5 seconds, 42 milliseconds)
[info] java.lang.AssertionError: assertion failed: expected no StreamSupervisor children, but got [Actor[pekko://UntypedMqttFlowSpec/system/Materializers/StreamSupervisor-3/flow-7-0-ignoreSink#-1480604319], Actor[pekko://UntypedMqttFlowSpec/system/Materializers/StreamSupervisor-3/flow-8-0-unnamed#1503904569]]
[info] at scala.Predef$.assert(Predef.scala:223)
[info] at org.apache.pekko.stream.testkit.scaladsl.StreamTestKit$.$anonfun$assertNoChildren$2(StreamTestKit.scala:64)
[info] at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
[info] at org.apache.pekko.testkit.TestKitBase.awaitAssert(TestKit.scala:342)
[info] at org.apache.pekko.testkit.TestKitBase.awaitAssert$(TestKit.scala:331)
[info] at org.apache.pekko.testkit.TestKit.awaitAssert(TestKit.scala:982)
[info] at org.apache.pekko.stream.testkit.scaladsl.StreamTestKit$.$anonfun$assertNoChildren$1(StreamTestKit.scala:61)
[info] at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
[info] at org.apache.pekko.testkit.TestKitBase.within(TestKit.scala:429)
[info] at org.apache.pekko.testkit.TestKitBase.within$(TestKit.scala:416)
[info] at org.apache.pekko.testkit.TestKit.within(TestKit.scala:982)
[info] at org.apache.pekko.testkit.TestKitBase.within(TestKit.scala:444)
[info] at org.apache.pekko.testkit.TestKitBase.within$(TestKit.scala:444)
[info] at org.apache.pekko.testkit.TestKit.within(TestKit.scala:982)
[info] at org.apache.pekko.stream.testkit.scaladsl.StreamTestKit$.assertNoChildren(StreamTestKit.scala:61)
[info] at org.apache.pekko.stream.testkit.scaladsl.StreamTestKit$.assertAllStagesStopped(StreamTestKit.scala:43)
[info] at docs.scaladsl.MqttFlowSpecBase.$anonfun$new$2(MqttFlowSpec.scala:65)
[info] at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
[info] at org.scalatest.OutcomeOf.outcomeOf(OutcomeOf.scala:85)
[info] at org.scalatest.OutcomeOf.outcomeOf$(OutcomeOf.scala:83)
[info] at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
[info] at org.scalatest.Transformer.apply(Transformer.scala:22)
[info] at org.scalatest.Transformer.apply(Transformer.scala:20)
[info] at org.scalatest.wordspec.AnyWordSpecLike$$anon$3.apply(AnyWordSpecLike.scala:1240)
[info] at org.apache.pekko.stream.connectors.testkit.scaladsl.LogCapturing.withFixture(LogCapturing.scala:70)
[info] at org.apache.pekko.stream.connectors.testkit.scaladsl.LogCapturing.withFixture$(LogCapturing.scala:66)
[info] at docs.scaladsl.MqttFlowSpecBase.withFixture(MqttFlowSpec.scala:48)
[info] at org.scalatest.wordspec.AnyWordSpecLike.invokeWithFixture$1(AnyWordSpecLike.scala:1238)
[info] at org.scalatest.wordspec.AnyWordSpecLike.$anonfun$runTest$1(AnyWordSpecLike.scala:1250)
[info] at org.scalatest.SuperEngine.runTestImpl(Engine.scala:306)
[info] at org.scalatest.wordspec.AnyWordSpecLike.runTest(AnyWordSpecLike.scala:1250)
[info] at org.scalatest.wordspec.AnyWordSpecLike.runTest$(AnyWordSpecLike.scala:1232)
[info] at docs.scaladsl.MqttFlowSpecBase.runTest(MqttFlowSpec.scala:48)
[info] at org.scalatest.wordspec.AnyWordSpecLike.$anonfun$runTests$1(AnyWordSpecLike.scala:1309)
[info] at org.scalatest.SuperEngine.$anonfun$runTestsInBranch$1(Engine.scala:413)
[info] at scala.collection.immutable.List.foreach(List.scala:431)
[info] at org.scalatest.SuperEngine.traverseSubNodes$1(Engine.scala:401)
[info] at org.scalatest.SuperEngine.runTestsInBranch(Engine.scala:390)
[info] at org.scalatest.SuperEngine.$anonfun$runTestsInBranch$1(Engine.scala:427)
[info] at scala.collection.immutable.List.foreach(List.scala:431)
[info] at org.scalatest.SuperEngine.traverseSubNodes$1(Engine.scala:401)
[info] at org.scalatest.SuperEngine.runTestsInBranch(Engine.scala:396)
[info] at org.scalatest.SuperEngine.runTestsImpl(Engine.scala:475)
[info] at org.scalatest.wordspec.AnyWordSpecLike.runTests(AnyWordSpecLike.scala:1309)
[info] at org.scalatest.wordspec.AnyWordSpecLike.runTests$(AnyWordSpecLike.scala:1308)
[info] at docs.scaladsl.MqttFlowSpecBase.runTests(MqttFlowSpec.scala:48)
[info] at org.scalatest.Suite.run(Suite.scala:1114)
[info] at org.scalatest.Suite.run$(Suite.scala:1096)
[info] at docs.scaladsl.MqttFlowSpecBase.org$scalatest$wordspec$AnyWordSpecLike$$super$run(MqttFlowSpec.scala:48)
[info] at org.scalatest.wordspec.AnyWordSpecLike.$anonfun$run$1(AnyWordSpecLike.scala:1354)
[info] at org.scalatest.SuperEngine.runImpl(Engine.scala:535)
[info] at org.scalatest.wordspec.AnyWordSpecLike.run(AnyWordSpecLike.scala:1354)
[info] at org.scalatest.wordspec.AnyWordSpecLike.run$(AnyWordSpecLike.scala:1352)
[info] at docs.scaladsl.MqttFlowSpecBase.org$scalatest$BeforeAndAfterAll$$super$run(MqttFlowSpec.scala:48)
[info] at org.scalatest.BeforeAndAfterAll.liftedTree1$1(BeforeAndAfterAll.scala:213)
[info] at org.scalatest.BeforeAndAfterAll.run(BeforeAndAfterAll.scala:210)
[info] at org.scalatest.BeforeAndAfterAll.run$(BeforeAndAfterAll.scala:208)
[info] at docs.scaladsl.MqttFlowSpecBase.run(MqttFlowSpec.scala:48)
[info] at org.scalatest.tools.Framework.org$scalatest$tools$Framework$$runSuite(Framework.scala:321)
[info] at org.scalatest.tools.Framework$ScalaTestTask.execute(Framework.scala:517)
[info] at sbt.TestRunner.runTest$1(TestFramework.scala:153)
[info] at sbt.TestRunner.run(TestFramework.scala:168)
[info] at sbt.TestFramework$$anon$3$$anonfun$$lessinit$greater$1.$anonfun$apply$1(TestFramework.scala:336)
[info] at sbt.TestFramework$.sbt$TestFramework$$withContextLoader(TestFramework.scala:296)
[info] at sbt.TestFramework$$anon$3$$anonfun$$lessinit$greater$1.apply(TestFramework.scala:336)
[info] at sbt.TestFramework$$anon$3$$anonfun$$lessinit$greater$1.apply(TestFramework.scala:336)
[info] at sbt.TestFunction.apply(TestFramework.scala:348)
[info] at sbt.Tests$.processRunnable$1(Tests.scala:475)
[info] at sbt.Tests$.$anonfun$makeSerial$1(Tests.scala:481)
[info] at sbt.std.Transform$$anon$3.$anonfun$apply$2(Transform.scala:47)
[info] at sbt.std.Transform$$anon$4.work(Transform.scala:69)
[info] at sbt.Execute.$anonfun$submit$2(Execute.scala:283)
[info] at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:24)
[info] at sbt.Execute.work(Execute.scala:292)
[info] at sbt.Execute.$anonfun$submit$1(Execute.scala:283)
[info] at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
[info] at sbt.CompletionService$$anon$2.call(CompletionService.scala:65)
[info] at java.util.concurrent.FutureTask.run(FutureTask.java:266)
[info] at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
[info] at java.util.concurrent.FutureTask.run(FutureTask.java:266)
[info] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
[info] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
[info] at java.lang.Thread.run(Thread.java:750)
```

Start with the failing UntypedMqttFlowSpec test and the shared setup in MqttFlowSpec.scala around line 65, then run that MQTT flow test to reproduce the leftover StreamSupervisor children. Done means the bidirectional connection and topic subscription test passes without leaving those children running.

Written by the indexing model from the issue text.