pubsublite.cloudpubsub.internal.SinglePartitionSubscriberTest: singleMessageNackHandlerFailedFuture failed

Note: #1386 was also for this test, but it was closed more than 10 days ago. So, I didn't mark it flaky.

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commit: 27cceaa2a97279fc6d7b65e849df909135d4aaa1
buildURL: [Build Status](https://source.cloud.google.com/results/invocations/1ad6c9b2-dc18-42e2-baf6-b559f373dade), [Sponge](http://sponge2/1ad6c9b2-dc18-42e2-baf6-b559f373dade)
status: failed
Test output

java.lang.IllegalStateException: failureCause() is only valid if the service has failed, service is RUNNING

	at com.google.common.base.Preconditions.checkState(Preconditions.java:601)

	at com.google.common.util.concurrent.AbstractService$StateSnapshot.failureCause(AbstractService.java:630)

	at com.google.common.util.concurrent.AbstractService.failureCause(AbstractService.java:499)

	at com.google.api.core.AbstractApiService.failureCause(AbstractApiService.java:111)

	at com.google.cloud.pubsublite.cloudpubsub.internal.SinglePartitionSubscriberTest.singleMessageNackHandlerFailedFuture(SinglePartitionSubscriberTest.java:230)

	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)

	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)

	at java.base/java.lang.reflect.Method.invoke(Method.java:566)

	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)

	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)

	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)

	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)

	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)

	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)

	at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)

	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)

	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)

	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)

	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)

	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)

	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)

	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)

	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)

	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)

	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)

	at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:316)

	at org.apache.maven.surefire.junit4.JUnit4Provider.executeWithRerun(JUnit4Provider.java:240)

	at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:214)

	at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:155)

	at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:385)

	at org.apache.maven.surefire.booter.ForkedBooter.execute(ForkedBooter.java:162)

	at org.apache.maven.surefire.booter.ForkedBooter.run(ForkedBooter.java:507)

	at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:495)