test: Add phpunit test suite to the plugin

[ahuseyn]

Add the unit tests to HWP Previews plugin with phpunit.

[colinmurphy]

@ahuseyn

Notes on what I am adding as part of the tests

- What are we testing?
  -Hooks
    - All actions and filters in ACTION_AND_FILTERS.md
    - Activation and deactivation hooks
- Preview URL generation
  - enabled
    - yes
    - no
  - iframe
    - enabled
    - disabled
  - post parent manager
    - different status for parent to child
    - same status for parent to child
  - post status
    - post status allowd
    - post status not allowed
  - Preview URL resolver
    - For all different parameters
    - Add a custom parameter and using this as part of the URL generation
  - REST API endpoint