test: refactor inspector-helper.js

There are two instances of `setTimeout()` called without a duration in
`inspector-helper.js`. Change to `setImmediate()` for clarity that it
isn't a mistake.

PR-URL: https://github.com/nodejs/node/pull/9499


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>