DockerContainerTests heseinbug

Failed in this Travis CI build 307526229

```
whisk.core.containerpool.docker.test.DockerContainerTests > DockerContainer should eventually terminate even if no sentinels can be found FAILED
org.scalatest.exceptions.TestFailedException: 246 was greater than or equal to 100, but 246 was not less than 200
at org.scalatest.MatchersHelper$.indicateFailure(MatchersHelper.scala:340)
at org.scalatest.Matchers$ShouldMethodHelper$.shouldMatcher(Matchers.scala:6668)
at org.scalatest.Matchers$AnyShouldWrapper.should(Matchers.scala:6704)
at whisk.core.containerpool.docker.test.DockerContainerTests$$anonfun$22.apply(DockerContainerTests.scala:582)
at whisk.core.containerpool.docker.test.DockerContainerTests$$anonfun$22.apply(DockerContainerTests.scala:560)
at org.scalatest.OutcomeOf$class.outcomeOf(OutcomeOf.scala:85)
at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
at org.scalatest.Transformer.apply(Transformer.scala:22)
at org.scalatest.Transformer.apply(Transformer.scala:20)
at org.scalatest.FlatSpecLike$$anon$1.apply(FlatSpecLike.scala:1682)
at org.scalatest.TestSuite$class.withFixture(TestSuite.scala:196)
at whisk.core.containerpool.docker.test.DockerContainerTests.org$scalamock$scalatest$AbstractMockFactory$$super$withFixture(DockerContainerTests.scala:59)
at org.scalamock.scalatest.AbstractMockFactory$$anonfun$withFixture$1.apply(AbstractMockFactory.scala:35)
at org.scalamock.scalatest.AbstractMockFactory$$anonfun$withFixture$1.apply(AbstractMockFactory.scala:34)
at org.scalamock.MockFactoryBase$class.withExpectations(MockFactoryBase.scala:51)
at whisk.core.containerpool.docker.test.DockerContainerTests.withExpectations(DockerContainerTests.scala:59)
at org.scalamock.scalatest.AbstractMockFactory$class.withFixture(AbstractMockFactory.scala:34)
at whisk.core.containerpool.docker.test.DockerContainerTests.withFixture(DockerContainerTests.scala:59)
at org.scalatest.FlatSpecLike$class.invokeWithFixture$1(FlatSpecLike.scala:1679)
at org.scalatest.FlatSpecLike$$anonfun$runTest$1.apply(FlatSpecLike.scala:1692)
at org.scalatest.FlatSpecLike$$anonfun$runTest$1.apply(FlatSpecLike.scala:1692)
at org.scalatest.SuperEngine.runTestImpl(Engine.scala:289)
at org.scalatest.FlatSpecLike$class.runTest(FlatSpecLike.scala:1692)
at whisk.core.containerpool.docker.test.DockerContainerTests.org$scalatest$BeforeAndAfterEach$$super$runTest(DockerContainerTests.scala:59)
at org.scalatest.BeforeAndAfterEach$class.runTest(BeforeAndAfterEach.scala:221)
at whisk.core.containerpool.docker.test.DockerContainerTests.runTest(DockerContainerTests.scala:59)
at org.scalatest.FlatSpecLike$$anonfun$runTests$1.apply(FlatSpecLike.scala:1750)
at org.scalatest.FlatSpecLike$$anonfun$runTests$1.apply(FlatSpecLike.scala:1750)
at org.scalatest.SuperEngine$$anonfun$traverseSubNodes$1$1.apply(Engine.scala:396)
at org.scalatest.SuperEngine$$anonfun$traverseSubNodes$1$1.apply(Engine.scala:384)
at scala.collection.immutable.List.foreach(List.scala:392)
at org.scalatest.SuperEngine.traverseSubNodes$1(Engine.scala:384)
at org.scalatest.SuperEngine.org$scalatest$SuperEngine$$runTestsInBranch(Engine.scala:373)
at org.scalatest.SuperEngine$$anonfun$traverseSubNodes$1$1.apply(Engine.scala:410)
at org.scalatest.SuperEngine$$anonfun$traverseSubNodes$1$1.apply(Engine.scala:384)
at scala.collection.immutable.List.foreach(List.scala:392)
at org.scalatest.SuperEngine.traverseSubNodes$1(Engine.scala:384)
at org.scalatest.SuperEngine.org$scalatest$SuperEngine$$runTestsInBranch(Engine.scala:379)
at org.scalatest.SuperEngine.runTestsImpl(Engine.scala:461)
at org.scalatest.FlatSpecLike$class.runTests(FlatSpecLike.scala:1750)
at org.scalatest.FlatSpec.runTests(FlatSpec.scala:1685)
at org.scalatest.Suite$class.run(Suite.scala:1147)
at org.scalatest.FlatSpec.org$scalatest$FlatSpecLike$$super$run(FlatSpec.scala:1685)
at org.scalatest.FlatSpecLike$$anonfun$run$1.apply(FlatSpecLike.scala:1795)
at org.scalatest.FlatSpecLike$$anonfun$run$1.apply(FlatSpecLike.scala:1795)
at org.scalatest.SuperEngine.runImpl(Engine.scala:521)
at org.scalatest.FlatSpecLike$class.run(FlatSpecLike.scala:1795)
at whisk.core.containerpool.docker.test.DockerContainerTests.org$scalatest$BeforeAndAfterAll$$super$run(DockerContainerTests.scala:59)
at org.scalatest.BeforeAndAfterAll$class.liftedTree1$1(BeforeAndAfterAll.scala:213)
at org.scalatest.BeforeAndAfterAll$class.run(BeforeAndAfterAll.scala:210)
at whisk.core.containerpool.docker.test.DockerContainerTests.run(DockerContainerTests.scala:59)
```

Start with DockerContainerTests.scala, especially the failing test around lines 560-582, and inspect the DockerContainer termination behavior it exercises. Reproduce the failure from Travis CI build 307526229 and determine why termination exceeds the test's 200-unit bound when no sentinels are found. Done means the test passes reliably while preserving eventual termination.

Written by the indexing model from the issue text.