Library.cs

namespace LibWithTests
{
    public class LibraryClass
    {
    }
}

LibWithTests.csproj

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

  <PropertyGroup>
    <TargetFramework Condition="$(Configuration)=='Test'">netcoreapp3.0</TargetFramework>
    <TargetFramework Condition="$(Configuration)!='Test'">netstandard2.0</TargetFramework>
    
    <IsPackable Condition="$(Configuration)=='Test'">false</IsPackable>
    <DefineConstants Condition="$(Configuration)=='Test'">TESTS</DefineConstants>
  </PropertyGroup>

  <ItemGroup Condition="$(Configuration)=='Test'">
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.2.0" />
    <PackageReference Include="xunit" Version="2.4.0" />
    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.0" />
    <PackageReference Include="coverlet.collector" Version="1.0.1" />
  </ItemGroup>

</Project>

UnitTest1.cs

using System;
#if TESTS
using Xunit;

namespace LibWithTests
{
    public class UnitTest1
    {
        [Fact]
        public void Test1()
        {

        }
    }
}
#endif