test: refactor test-init.js

1. Lot of repeating code has been refactored to a function
2. Errors in async calls are properly asserted
3. Fail the test if the callbacks are not fired

PR-URL: https://github.com/nodejs/node/pull/10384


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Julian Duque <julianduquej@gmail.com>