Adopt PHPUnit for plugin unit tests

tests/bootstrap.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace OPNsense\NordVPN;
+
+class Settings
+{
+    public object $general;
+    public object $managed;
+
+    public function __construct()
+    {
+        $this->general = (object)['countries' => ''];
+        $this->managed = (object)[
+            'server_country' => 'NL',
+            'server_hostname' => 'nl.example.invalid',
+        ];
+    }
+}