CSharp: added waiting for request_referencing and test for cross-file references

test/resources/repos/csharp/test_repo/Models/Person.cs

@@ -1,3 +1,5 @@
+using TestProject;
+
 namespace TestProject.Models
 {
     public class Person
@@ -22,5 +24,11 @@ namespace TestProject.Models
         {
             return Age >= 18;
         }
+        
+        public int CalculateYearsUntilRetirement()
+        {
+            var calculator = new Calculator();
+            return calculator.Subtract(65, Age);
+        }
     }
 }