Restore Repro

app.csproj

<Project Sdk="Microsoft.NET.Sdk">

	<PropertyGroup>
		<OutputType>Exe</OutputType>
		<TargetFramework>netcoreapp2.1</TargetFramework>
		<RootNamespace>App</RootNamespace>
		<GenerateAssemblyConfigurationAttribute>false</GenerateAssemblyConfigurationAttribute>
		<GenerateAssemblyCompanyAttribute>false</GenerateAssemblyCompanyAttribute>
		<GenerateAssemblyProductAttribute>false</GenerateAssemblyProductAttribute>
	</PropertyGroup>

	<ItemGroup>
		<Compile Update="./Program.cs"/>
	</ItemGroup>

</Project>

Program.cs

using System;

namespace App
{
	class Program
	{
		static void Main(String[] args)
		{
			var program = new Program();
			Console.WriteLine(program.Greeting());
		}

		public String Greeting()
		{
			return "Hello World!";
		}
	}
}

Test.cs

using Xunit;
using App;

namespace Tests
{
	public class ProgramTests
	{

		[Fact]
		public void ReturnsHello()
		{
			var program = new Program();
			var expectedGreeting = "hello";
			var actualGreeting = program.Greeting();
			Assert.True(expectedGreeting == actualGreeting);
		}
	}
}

tests.csproj

<Project Sdk="Microsoft.NET.Sdk">

	<PropertyGroup>
		<TargetFramework>netcoreapp2.1</TargetFramework>
		<RootNamespace>Tests</RootNamespace>
	</PropertyGroup>

	<ItemGroup>
		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.7.2" />
		<PackageReference Include="xunit" Version="2.3.1" />
	</ItemGroup>

	<ItemGroup>
		<ProjectReference Include="app.csproj" />
	</ItemGroup>

	<ItemGroup>
		<Compile Update="./Test.cs" />
	</ItemGroup>

</Project>