better csharp tests (#7727)

Author: wing328

modules/openapi-generator/src/main/resources/csharp-netcore/api_test.mustache

@@ -11,10 +11,11 @@ using Xunit;
 using {{packageName}}.Client;
 using {{packageName}}.{{apiPackage}};
 {{#hasImport}}
-using {{packageName}}.{{modelPackage}};
+// uncomment below to import models
+//using {{packageName}}.{{modelPackage}};
 {{/hasImport}}
 
-namespace {{packageName}}.Test
+namespace {{packageName}}.Test.Api
 {
     /// <summary>
     ///  Class for testing {{classname}}

modules/openapi-generator/src/main/resources/csharp-netcore/model_test.mustache

@@ -14,7 +14,7 @@ using Newtonsoft.Json;
 
 {{#models}}
 {{#model}}
-namespace {{packageName}}.Test
+namespace {{packageName}}.Test.Model
 {
     /// <summary>
     ///  Class for testing {{classname}}