Improve tests structure and test coverage

The test coverage reported was 97%, but it was quite closer to 0 than 100 if you include `@covers` annotations.

I've introduced unit tests to cover all of the functionality (except for the invalid JSON response) and now the test coverage is realistic. Scrutinizer CI shows ~~98.65%~~ 98.37% https://scrutinizer-ci.com/g/OpenBuildings/postmark/inspections/e3a3e0a8-c41e-482e-8353-b68931686c2c/code-structure/test-coverage

I've also improved the tests structure with better autoloading of helper classes.