Add a lifecyle API or a callback to detect/be notified when the engine has finished processing a script.

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The current implementation allows to determine if the engine has finished processing a script only if the script is guarantee to be synchronous.
We need a way to detect when an async script has exit the loop.

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One subtask is to trap the process.exit call in the test script, since it currently stops the jxcore engine so that it doesn't process the next test.
Quick fix for now: global automated replace of process.exit with return.

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Worked is finished, calling process.exit is not anymore an issue and it's handled correctly.
The code will be push along with other tasks regarding the restart-able thread.