Requirement
apex class calls webservice to retrieve product information. I have to schedule this class to run at night 10 pm everyday and write test class for the callout.
Solution:
I have written apex class which does the callout and upsert product object. Since I have to schedule this update i called the apex class from schedule class. As I cannot do synchronous callouts in schedule apex i have made the method as asychronous(@future) (asynchronous methods has no return type). When i have to achieve 100% code coverage for apex class which calls webservice i have to mock the callout and generate a fake response. The problem in this solution is that future methods has no return type and the test class that mocks the callout expects response.
Appreciate your responses.

Apex class:
global class Inventorycallout
{
    @future(callout=true)
    global static void inventorycall () 
    {
        Map<String, Object> m1 = new Map<String, Object>();
        List<Product2> productlist = new List<Product2>();
           
        Http http = new Http();    
        HttpRequest request = new HttpRequest();
              
        String endpoint = 'https://th-superbadge-apex.herokuapp.com/equipment';
        request.setMethod('GET');
        request.setEndpoint(endpoint);
        HttpResponse response = http.send(request);
        system.debug(response.getBody());
        List<Object> responselist = (List<Object>) JSON.deserializeUntyped(response.getBody());
        for(Object o : responselist)
        {
            Product2 pr = new Product2();
            m1 = (Map<String, Object>)o;
            pr.Replacement_Part__c= (Boolean)m1.get('replacement');
            pr.Current_Inventory__c = (Decimal)m1.get('quantity');
            pr.Name= (string)m1.get('name');
            pr.Maintenance_Cycle__c= (Decimal)m1.get('maintenanceperiod');
            pr.Lifespan_Months__c= (Decimal)m1.get('lifespan');
            pr.Warehouse_SKU__c= (string)m1.get('sku');
            productlist.add(pr);
        }
if(productlist.size() > 0)
        {
                Schema.SObjectField f = Product2.Fields.Warehouse_SKU__c;
                Database.UpsertResult [] cr =  Database.upsert(productlist,f,false);
}

Test class:
@isTest(seeallData = false)
private class inventorycallouttest
{    
   static testmethod void testrun()
   {
     Test.startTest();
      setMock(HttpCalloutMock.class,new MockHttpcall()); 
             HttpResponse response = Inventorycallout.inventorycall();
              String contentType = response.getHeader('Content-Type');
              System.assert(contentType == 'application/json');
              String actualValue = response.getBody();
              String expectedValue = '[{"fakeresponse":"test"},{"fakeresponse2":"test2"}]';
              System.assertEquals(actualValue, expectedValue);
              System.assertEquals(200, response.getStatusCode());
              Test.stopTest();          
   }
}