Extend verification test coverage (#3149)

Test NSwagClientExample in SwaggerIntegrationTests. Test Basic, CustomDocumentSerializer and NSwagClientExample in Verify tests.

Author: Saibamen

test/Swashbuckle.AspNetCore.IntegrationTests/SwaggerIntegrationTests.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Globalization;
 using System.Linq;
 using System.Net.Http;
@@ -27,6 +27,7 @@ public class SwaggerIntegrationTests
         [InlineData(typeof(GenericControllers.Startup), "/swagger/v1/swagger.json")]
         [InlineData(typeof(MultipleVersions.Startup), "/swagger/1.0/swagger.json")]
         [InlineData(typeof(MultipleVersions.Startup), "/swagger/2.0/swagger.json")]
+        [InlineData(typeof(NSwagClientExample.Startup), "/swagger/v1/swagger.json")]
         [InlineData(typeof(OAuth2Integration.Startup), "/resource-server/swagger/v1/swagger.json")]
         [InlineData(typeof(ReDocApp.Startup), "/swagger/v1/swagger.json")]
         [InlineData(typeof(TestFirst.Startup), "/swagger/v1-generated/openapi.json")]