Remove Thread.sleep() from port forward test class

[sarveshkaushal]

Summary

Replaces the fixed Thread.sleep() in PortForwardTest with semaphore-based synchronization.

The test needs to close the WebSocket handler only after PortForwardResult.init has created the input stream which it will block while reading the port identifier. A test-specific WebSocketStreamHandler subclass signals a semaphore when stream 0 is created, allowing the test to close the handler at the correct point.

A second semaphore waits for init() to finish after the handler is closed.

This is related to issue #1223

Testing

Ran mvn install on the root of the project.

[kubernetes-prow]

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This pull-request has been approved by: sarveshkaushal
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[sarveshkaushal]

/assign @brendandburns
/assign @yue9944882

[brendandburns]

As with the previous one please minimize diff for easier reviewing.