Initial attempt for Feature: expand verify to ensure no async assertions are pending.

[rpgeeganage]

Purpose (TL;DR) - mandatory

This PR is to get a clear idea about the feature mentioned below Feature: expand verify to ensure no async assertions are pending (https://github.com/sinonjs/referee/issues/53) (No tests were added yet. this pr is just to get an idea).

How to verify - mandatory

1. Check out this branch
2. npm install
3. npm test

Checklist for author

• [ ] npm run lint passes
• [ ] References to standard library functions are cached.

[rpgeeganage]

@mroderick, This is an initial attempt. Is this something you prefer as a solution? Thanks.