"Connection refused" in Flink/Spark python loopback tests

These error messages seem to be benign, but they're extremely annoying.

org.apache.beam.vendor.grpc.v1p26p0.io.grpc.StatusRuntimeException: UNAVAILABLE: io exception
at org.apache.beam.vendor.grpc.v1p26p0.io.grpc.stub.ClientCalls.toStatusRuntimeException(ClientCalls.java:240)
at org.apache.beam.vendor.grpc.v1p26p0.io.grpc.stub.ClientCalls.getUnchecked(ClientCalls.java:221)
at org.apache.beam.vendor.grpc.v1p26p0.io.grpc.stub.ClientCalls.blockingUnaryCall(ClientCalls.java:140)
at org.apache.beam.model.fnexecution.v1.BeamFnExternalWorkerPoolGrpc$BeamFnExternalWorkerPoolBlockingStub.stopWorker(BeamFnExternalWorkerPoolGrpc.java:247)
at org.apache.beam.runners.fnexecution.environment.ExternalEnvironmentFactory$1.close(ExternalEnvironmentFactory.java:159)
at org.apache.beam.runners.fnexecution.control.DefaultJobBundleFactory$WrappedSdkHarnessClient.$closeResource(DefaultJobBundleFactory.java:635)
at org.apache.beam.runners.fnexecution.control.DefaultJobBundleFactory$WrappedSdkHarnessClient.close(DefaultJobBundleFactory.java:635)
at org.apache.beam.runners.fnexecution.control.DefaultJobBundleFactory$WrappedSdkHarnessClient.unref(DefaultJobBundleFactory.java:651)
at org.apache.beam.runners.fnexecution.control.DefaultJobBundleFactory$WrappedSdkHarnessClient.access$400(DefaultJobBundleFactory.java:581)
at org.apache.beam.runners.fnexecution.control.DefaultJobBundleFactory.lambda$createEnvironmentCaches$3(DefaultJobBundleFactory.java:213)
at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.cache.LocalCache.processPendingNotifications(LocalCache.java:1809)
at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.cache.LocalCache$Segment.runUnlockedCleanup(LocalCache.java:3462)
at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.cache.LocalCache$Segment.postWriteCleanup(LocalCache.java:3438)
at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.cache.LocalCache$Segment.clear(LocalCache.java:3215)
at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.cache.LocalCache.clear(LocalCache.java:4270)
at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.cache.LocalCache$LocalManualCache.invalidateAll(LocalCache.java:4909)
at org.apache.beam.runners.fnexecution.control.DefaultJobBundleFactory.close(DefaultJobBundleFactory.java:320)
at org.apache.beam.runners.fnexecution.control.DefaultExecutableStageContext.close(DefaultExecutableStageContext.java:43)
at org.apache.beam.runners.fnexecution.control.ReferenceCountingExecutableStageContextFactory$WrappedContext.closeActual(ReferenceCountingExecutableStageContextFactory.java:208)
at org.apache.beam.runners.fnexecution.control.ReferenceCountingExecutableStageContextFactory$WrappedContext.access$200(ReferenceCountingExecutableStageContextFactory.java:184)
at org.apache.beam.runners.fnexecution.control.ReferenceCountingExecutableStageContextFactory.release(ReferenceCountingExecutableStageContextFactory.java:173)
at org.apache.beam.runners.fnexecution.control.ReferenceCountingExecutableStageContextFactory.lambda$scheduleRelease$1(ReferenceCountingExecutableStageContextFactory.java:127)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:295)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: org.apache.beam.vendor.grpc.v1p26p0.io.netty.channel.AbstractChannel$AnnotatedConnectException: Connection refused: localhost/0:0:0:0:0:0:0:1:38817
Caused by: java.net.ConnectException: Connection refused
at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method)
at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:717)
at org.apache.beam.vendor.grpc.v1p26p0.io.netty.channel.socket.nio.NioSocketChannel.doFinishConnect(NioSocketChannel.java:327)
at org.apache.beam.vendor.grpc.v1p26p0.io.netty.channel.nio.AbstractNioChannel$AbstractNioUnsafe.finishConnect(AbstractNioChannel.java:334)
at org.apache.beam.vendor.grpc.v1p26p0.io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:688)
at org.apache.beam.vendor.grpc.v1p26p0.io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:635)
at org.apache.beam.vendor.grpc.v1p26p0.io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:552)
at org.apache.beam.vendor.grpc.v1p26p0.io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:514)
at org.apache.beam.vendor.grpc.v1p26p0.io.netty.util.concurrent.SingleThreadEventExecutor$6.run(SingleThreadEventExecutor.java:1044)
at org.apache.beam.vendor.grpc.v1p26p0.io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
at org.apache.beam.vendor.grpc.v1p26p0.io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.lang.Thread.run(Thread.java:748)

Imported from Jira [BEAM-10264](https://issues.apache.org/jira/browse/BEAM-10264). Original Jira may contain additional context.
Reported by: ibzib.