Test: Clarify some test(file)s

* Rename `cancellation-test` to "scope" it under `promise`
* Clarify naming of `subclass-test`
* Use `sentinels` helper
* Clarify method names when describing tests
* Don't rely on `adapter` helper in our own tests
Test: Add util file

* Include method to test correct promise constructor behavior
`blessed.extended()` helper to extend promises
Helper to guard supposed promises-for-arrays composition
Collection subclass with sugar for promises-for-arrays
Sugar for promise-resolution races
Sugar for promises
Sugar for delays and timeouts
Sugar for managing concurrency
Sugar for functional applications
Update Readme, give credit for the sugar