Tests: add unit test workflow

[glaubersilva]

Resolves #73

Description

This PR implements a workflow to automatically run the PHPUnit tests of the add-on.

For now, the tests aren't passing because one is failing in every scenario and others are failing just with PHP 8.

So, for a temporary test, I edited the test that is failing in all scenarios just to check how the workflow will work.

And here is the result: https://github.com/impress-org/givewp-next-gen/actions/runs/3297201959/jobs/5437950344

Affects

The "push" and "pull request" operations for this repository.

Visuals

Testing Instructions

Just push something (or open a pull request like this) to the develop or master branches

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Relevant @unreleased tags included in DocBlocks
• Includes unit tests
• Reviewed by the designer (if follows a design)
• Self Review of code and UX completed

[jonwaldstein]

@glaubersilva great work!

I updated this PR to:

1. Re-arrange the order of the steps slightly so that we're checking out both the repositories first
2. Added a step to setup node and run the build process - I found this was needed for feature tests.
3. Updated any failing unit Tests

At this point i'll move the workflow yml file into our github actions repo and then just reference it here.

[glaubersilva]

@jonwaldstein Great! Glad to see it working. =)