Improve PHPUnit and some testing stuffs

[peter279k]

Changed log

• Removing the use Ibericode\Vat\Clients\JsonVatClient; namespace because it's declared but not used.
• Switching the arguments order for assertEquals assertion because the arguments order is assertEquals($expected, $actual).
• The PHPUnit assertion call approaches are $this and self. Most of them use $this and it should be consistency for them to use the $this approach.
• It should be good to use the assertCount to assert the $countries counts are 245.
• According to the PHPUnit doc, it should use the protected function setUp(): void method.

[dannyvankooten]

:clap: Awesome work, thank you @peter279k!