fix: prefill correct value for country in dummy data

tests/Service/ParticipantDataPrefillerTest.php

@@ -276,6 +276,7 @@ class ParticipantDataPrefillerTest extends TestCase
         $this->assertSame('[email protected]', $participant->email);
         $this->assertSame('2006-01-15', $participant->dateOfBirth?->format('Y-m-d'));
         $this->assertSame('Musterstr. 123', $participant->address?->street);
+        $this->assertSame('DE', $participant->address?->country);
 
         CarbonImmutable::setTestNow();
     }