Test flaky failure observed

[java-codehunger]

Describe the bug
I ran this test for many many times in normal settings and got the following failures for many times.

Debug log
Running org.java_websocket.issues.Issue580Test
WebSocketConnectReadThread-41
at java.base@11.0.17/java.net.SocketInputStream.socketRead0(Native Method)
at java.base@11.0.17/java.net.SocketInputStream.socketRead(SocketInputStream.java:115)
at java.base@11.0.17/java.net.SocketInputStream.read(SocketInputStream.java:168)
at java.base@11.0.17/java.net.SocketInputStream.read(SocketInputStream.java:140)
at java.base@11.0.17/java.net.SocketInputStream.read(SocketInputStream.java:126)
at app//org.java_websocket.client.WebSocketClient.run(WebSocketClient.java:515)
at java.base@11.0.17/java.lang.Thread.run(Thread.java:829)

WebSocketWriteThread-42
at java.base@11.0.17/jdk.internal.misc.Unsafe.park(Native Method)
at java.base@11.0.17/java.util.concurrent.locks.LockSupport.park(LockSupport.java:194)
at java.base@11.0.17/java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2081)
at java.base@11.0.17/java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:433)
at app//org.java_websocket.client.WebSocketClient$WebsocketWriteThread.runWriteData(WebSocketClient.java:816)
at app//org.java_websocket.client.WebSocketClient$WebsocketWriteThread.run(WebSocketClient.java:799)
at java.base@11.0.17/java.lang.Thread.run(Thread.java:829)

connectionLostChecker-1
at java.base@11.0.17/jdk.internal.misc.Unsafe.park(Native Method)
at java.base@11.0.17/java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:234)
at java.base@11.0.17/java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2123)
at java.base@11.0.17/java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1182)
at java.base@11.0.17/java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:899)
at java.base@11.0.17/java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1054)
at java.base@11.0.17/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1114)
at java.base@11.0.17/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base@11.0.17/java.lang.Thread.run(Thread.java:829)
[ERROR] Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 1.48 s <<< FAILURE! - in org.java_websocket.issues.Issue580Test
[ERROR] runNoCloseBlockingTestScenario7 Time elapsed: 1.44 s <<< FAILURE!
java.lang.AssertionError: Found 3 zombie thread(s)

[INFO]
[INFO] Results:
[INFO]
[ERROR] Failures:
[ERROR] Issue580Test.runNoCloseBlockingTestScenario[7] Found 3 zombie thread(s)
[INFO]
[ERROR] Tests run: 1, Failures: 1, Errors: 0, Skipped: 0

Steps to reproduce the behavior:
I run the following command many times without modifying anything.

mvn test -Dtest=org.java_websocket.issues.Issue580Test#runNoCloseBlockingTestScenario[7]

This test is also possible to make consistently fail if we inject 100ms delay before the line 496 of the Draft_6455.java

Expected behavior:
Test failure should not occur

Environment(please complete the following information):
I ran the test on an Ubuntu 20.04 LTS machine using OpenJDK 1.8.0_312.