Test WorkerGlobalScope.close() method. Will print PASS multiple times, followed by DONE.

Test 1: Call close() in the worker, then enter infinite loop. From the Worker object side, call terminate(). Verify the loop is terminated.
PASS: workerThreadCount = 0
PASS: Worker thread created and closed, received message: closed
PASS: Worker exited after terminate() terminated the infinite loop.
Test 2: post 2 messages to a MessagePort into the worker. port.onmessage handler inside the worker will call close(). Verify that the second onmessage was not executed.
PASS: workerThreadCount = 0
PASS: MessagePort multiple message test, received message: echo_ping1
PASS: MessagePort messages were not delivered asfter close().
DONE