Updates Behat ~3.0 | Fresh behat --init | Simple behat.yml

- Deletes existing Behat related code
Adds FunctionalFeatureContext and 1st Feature w/ Complete Scenario

- Adds Feature: The "testing" endpoint
    - Adds Scenario: the most basic request
- FunctionalFeatureContext
    - Adds GuzzleHttp\Client and --append-snippets
    - Uses PHPUnit assertions
    - Renames arguments
    - Implements all methods.
Adds "testing" endpoint/controller. Implements @index. Tests passing!