Flink validatesPortableRunner* tasks failing locally

Despite passing on Jenkins [1], portable Flink tests are failing on my linux desktop. The tests that fail don't seem to have any correlation. I suspect a race condition in Flink classloading.

Tests that failed on my example run (Lara reported different tests though so I'm guessing they're basically random):

MetricsPusherTest. pushesSystemMetrics
MetricsPusherTest. pushesUserMetrics
WithTimestampsTest. withTimestampsBackwardsInTimeAndWithAllowedTimestampSkewShouldSucceed
WithTimestampsTest. withTimestampsLambdaShouldApplyTimestamps
CoGroupByKeyTest. testCoGroupByKey
CoGroupByKeyTest. testCoGroupByKeyGetOnly
CoGroupByKeyTest. testCoGroupByKeyHandleResults
CoGroupByKeyTest. testCoGroupByKeyWithWindowing
WindowTest. testNoWindowFnDoesNotReassignWindows
WindowTest. testTimestampCombinerDefault
WindowTest. testTimestampCombinerEndOfWindow
WindowingTest. testMergingWindowing
WindowingTest. testNonPartitioningWindowing
WindowingTest. testPartitioningWindowing
WindowingTest. testWindowPreservation
PCollectionTupleTest. testComposePCollectionTuple

Errors look like this:

java.lang.IllegalArgumentException: No classpath elements found.
at org.apache.beam.runners.portability.PortableRunner.run(PortableRunner.java:136)
at org.apache.beam.runners.portability.testing.TestPortableRunner.run(TestPortableRunner.java:80)
at org.apache.beam.sdk.Pipeline.run(Pipeline.java:323)
at org.apache.beam.sdk.testing.TestPipeline.run(TestPipeline.java:399)
at org.apache.beam.sdk.testing.TestPipeline.run(TestPipeline.java:335)
at org.apache.beam.sdk.transforms.WithTimestampsTest.withTimestampsLambdaShouldApplyTimestamps(WithTimestampsTest.java:206)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
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.apache.beam.sdk.testing.TestPipeline$1.evaluate(TestPipeline.java:323)
at org.junit.rules.ExpectedException$ExpectedExceptionStatement.evaluate(ExpectedException.java:258)
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$3.evaluate(ParentRunner.java:306)
at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.runTestClass(JUnitTestClassExecutor.java:110)
at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.execute(JUnitTestClassExecutor.java:58)
at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.execute(JUnitTestClassExecutor.java:38)
at org.gradle.api.internal.tasks.testing.junit.AbstractJUnitTestClassProcessor.processTestClass(AbstractJUnitTestClassProcessor.java:62)
at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.processTestClass(SuiteTestClassProcessor.java:51)
at sun.reflect.GeneratedMethodAccessor106.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:36)
at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)
at org.gradle.internal.dispatch.ContextClassLoaderDispatch.dispatch(ContextClassLoaderDispatch.java:33)
at org.gradle.internal.dispatch.ProxyDispatchAdapter$DispatchingInvocationHandler.invoke(ProxyDispatchAdapter.java:94)
at com.sun.proxy.$Proxy2.processTestClass(Unknown Source)
at org.gradle.api.internal.tasks.testing.worker.TestWorker$2.run(TestWorker.java:176)
at org.gradle.api.internal.tasks.testing.worker.TestWorker.executeAndMaintainThreadName(TestWorker.java:129)
at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:100)
at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:60)
at org.gradle.process.internal.worker.child.ActionExecutionWorker.execute(ActionExecutionWorker.java:56)
at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:133)
at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:71)
at worker.org.gradle.process.internal.worker.GradleWorkerMain.run(GradleWorkerMain.java:69)
at worker.org.gradle.process.internal.worker.GradleWorkerMain.main(GradleWorkerMain.java:74)

./gradlew :runners:flink:1.13:job-server:validatesPortableRunnerStreaming

[1] https://ci-beam.apache.org/job/beam_PostCommit_Java_PVR_Flink_Streaming

Imported from Jira [BEAM-13600](https://issues.apache.org/jira/browse/BEAM-13600). Original Jira may contain additional context.
Reported by: ibzib.