INT 283 - Add tests to improve maintainability

[joaokamun]

This PR adds tests to improve maintainability of our codebase. We defined previously that we're going to use Pest PHP and our minimum php support will be 7.3.

This PR don't fully coverage our code yet.

Also want to highlight the work of @roberto-butti, that was base of many parts of this PR.

Pull request type

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [x] Other (please describe):

How to test this PR

Run our tests: ./vendor/bin/pest

[roberto-butti]

You are doing a great job. After introducing tests, we will introduce also static code analysis with PHPstan ... 🚀 🚀 🚀

[joaokamun]

This PR also fixes: #19