Hi Mine code is working fine and passed the trail, please have a look :-

Step 1 - First generated the Apex classes from Generate from WSDL

//Generated by wsdl2apex

public class ParkService {
    public class byCountryResponse {
        public String[] return_x;
        private String[] return_x_type_info = new String[]{'return','http://parks.services/',null,'0','-1','false'};
        private String[] apex_schema_type_info = new String[]{'http://parks.services/','false','false'};
        private String[] field_order_type_info = new String[]{'return_x'};
    }
    public class byCountry {
        public String arg0;
        private String[] arg0_type_info = new String[]{'arg0','http://parks.services/',null,'0','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://parks.services/','false','false'};
        private String[] field_order_type_info = new String[]{'arg0'};
    }
    public class ParksImplPort {
        public String endpoint_x = 'https://th-apex-soap-service.herokuapp.com/service/parks';
        public Map<String,String> inputHttpHeaders_x;
        public Map<String,String> outputHttpHeaders_x;
        public String clientCertName_x;
        public String clientCert_x;
        public String clientCertPasswd_x;
        public Integer timeout_x;
        private String[] ns_map_type_info = new String[]{'http://parks.services/', 'ParkService'};
        public String[] byCountry(String arg0) {
            ParkService.byCountry request_x = new ParkService.byCountry();
            request_x.arg0 = arg0;
            ParkService.byCountryResponse response_x;
            Map<String, ParkService.byCountryResponse> response_map_x = new Map<String, ParkService.byCountryResponse>();
            response_map_x.put('response_x', response_x);
            WebServiceCallout.invoke(
              this,
              request_x,
              response_map_x,
              new String[]{endpoint_x,
              '',
              'http://parks.services/',
              'byCountry',
              'http://parks.services/',
              'byCountryResponse',
              'ParkService.byCountryResponse'}
            );
            response_x = response_map_x.get('response_x');
            return response_x.return_x;
        }
    }
}
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Step - 2 
------------------------------------------------------------------------------------
public class ParkLocator {
    public static String[] country(String ctry) {
        ParkService.ParksImplPort park = new ParkService.ParksImplPort();
        return park.byCountry(ctry);
    }
}
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Step 3
---------------------------------------------------------------------------------------
@isTest
global class ParkServiceMock implements WebServiceMock {
   global void doInvoke(
           Object stub,
           Object request,
           Map<String, Object> response,
           String endpoint,
           String soapAction,
           String requestName,
           String responseNS,
           String responseName,
           String responseType) {
        // start - specify the response you want to send
        ParkService.byCountryResponse response_x = new ParkService.byCountryResponse();
        response_x.return_x = new List<String>{'Hamburg Wadden Sea National Park', 'Hainich National Park', 'Bavarian Forest National Park'};
       
        response.put('response_x', response_x);
   }
}
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Step 4
----------------------------------
@isTest
private class ParkLocatorTest {
    @isTest public static void testCallout() {             
        // This causes a fake response to be generated
        Test.setMock(WebServiceMock.class, new ParkServiceMock());
             
        String country = 'Germany';
        String[] result = ParkLocator.Country(country);
          System.debug(result);
        String resultEx ='Hamburg Wadden Sea National Park, Hainich National Park, Bavarian Forest National Park';
        System.assertEquals(resultEx,resultEx);
            }
}
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Happy Coding :0