Add separate project for tests

csharpcore/GuildedRoseTests/ApprovalTest.cs

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+using Xunit;
+using System;
+using System.IO;
+using System.Text;
+using ApprovalTests;
+using ApprovalTests.Reporters;
+using GuildedRose;
+
+namespace GuildedRoseTests
+{
+    [UseReporter(typeof(DiffReporter))]
+    public class ApprovalTest
+    {
+        [Fact]
+        public void ThirtyDays()
+        {
+            var fakeoutput = new StringBuilder();
+            Console.SetOut(new StringWriter(fakeoutput));
+            Console.SetIn(new StringReader("a\n"));
+
+            Program.Main(new string[] { });
+            var output = fakeoutput.ToString();
+
+            Approvals.Verify(output);
+        }
+    }
+}

csharpcore/GuildedRoseTests/GildedRoseTest.cs

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+using Xunit;
+using System.Collections.Generic;
+using GuildedRose;
+
+namespace GuildedRoseTests
+{
+    public class GildedRoseTest
+    {
+        [Fact]
+        public void foo()
+        {
+            IList<Item> Items = new List<Item> { new Item { Name = "foo", SellIn = 0, Quality = 0 } };
+            GildedRose app = new GildedRose(Items);
+            app.UpdateQuality();
+            Assert.Equal("fixme", Items[0].Name);
+        }
+    }
+}

csharpcore/GuildedRoseTests/GuildedRoseTests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="ApprovalTests" Version="5.5.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.10.0" />
+    <PackageReference Include="xunit" Version="2.4.1" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
+    <PackageReference Include="coverlet.collector" Version="3.1.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\GuildedRose\GuildedRose.csproj" />
+  </ItemGroup>
+
+</Project>