Add PHP unit test

Looks good to me,
- patch applied,
- code looks good,
- test checks adding the country field and testing the page after save,
extra points for the autocomplete,
- phpcs passes,
- phpunit runs without errors.

One minor note:

    // Disable strict config schema checking for this test.
    $this->strictConfigSchema = FALSE;

don't think this is needed, and the test also pass without it.

Veel dank!