fix: Treats natural node.js process termination as exit code 0

[jakobo]

Issue

node.js natural process termination triggered an exit code of 128, caused by the asynchronous beforeExit event.

Fix

Changes the asynchronous beforeExit to trigger with an exit code 0, mirroring the behavior used the PM2 event

Summary of Changes - patch level change

• Moved fixtures to a /fixtures directory (when you create the third fixture, you'll probably need the 4th down the line and this makes it clear where they go)
• Added empty fixture that replicates the bug reported in 23
• Resolves issue in index.js

Fixes #23