test: enhance C# test case with more language features and symbols

tests/basic/test_repomap.py

@@ -598,11 +598,46 @@ export { UserGreeting, useCounter, DEFAULT_NAME, MAX_AGE };
             ),
             "csharp": (
                 "test.cs",
-                (
-                    "using System;\n\nclass Program {\n    static void Main() {\n       "
-                    ' Console.WriteLine("Hello, World!");\n    }\n}\n'
-                ),
-                "Program",  # Key symbol to check
+                """using System;
+using System.Collections.Generic;
+
+namespace Greetings {
+    public interface IGreeter {
+        string Greet(string name);
+    }
+
+    public class Person {
+        public string Name { get; set; }
+        public int Age { get; set; }
+
+        public Person(string name, int age) {
+            Name = name;
+            Age = age;
+        }
+    }
+
+    public class FormalGreeter : IGreeter {
+        private const string PREFIX = "Good day";
+        private static readonly int MAX_AGE = 150;
+
+        public string Greet(string name) {
+            return $"{PREFIX}, {name}!";
+        }
+
+        public string GreetPerson(Person person) {
+            return $"{PREFIX}, {person.Name} ({person.Age})!";
+        }
+    }
+
+    public class Program {
+        static void Main() {
+            var greeter = new FormalGreeter();
+            var person = new Person("World", 42);
+            Console.WriteLine(greeter.GreetPerson(person));
+        }
+    }
+}""",
+                "IGreeter",  # Key symbol to check
             ),
             #####################
             "dart": (