Add approval test to csharpcore

csharpcore/GildedRoseTests/ApprovalTest.cs

+using System;
+using System.IO;
+using System.Text;
+using ApprovalTests;
+using ApprovalTests.Reporters;
+using GildedRoseTests;
+using NUnit.Framework;
+
+namespace csharp
+{
+    [UseReporter(typeof(DiffReporter))]
+    [TestFixture]
+    public class ApprovalTest
+    {
+        [Test]
+        public void ThirtyDays()
+        {
+            
+            StringBuilder fakeoutput = new StringBuilder();
+            Console.SetOut(new StringWriter(fakeoutput));
+            Console.SetIn(new StringReader("a\n"));
+
+            TexttestFixture.Main(new string[] { });
+            var output = fakeoutput.ToString();
+
+            Approvals.Verify(output);
+        }
+    }
+}
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