Integration and unit tests no longer work on ASP.NET Core 2.1 failing to find assemblies at runtime

Update: This has been made easier with 2.2 Tooling. Make sure that your dotnet --version SDK version is at least 2.2.100, even when buidling 2.1 applications

Just add a versionless package reference to your project while keeping the Microsoft.NET.Sdk:

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

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

      <ItemGroup>
        <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="2.1.1" />
        <PackageReference Include="Microsoft.AspNetCore.App" />
        <!-- other references to xunit, test SDK etc. -->
      </ItemGroup>

      <ItemGroup>
        <ProjectReference Include="..\AspNetCoreAppToTest\AspNetCoreAppToTest.csproj" />
      </ItemGroup>

    </Project>

Original:

ASP.NET Core 2.1 uses a new "shared framework" to run ASP.NET Core applications on. Test projects need to be modified/updated to also use this shared framework using one of the following approaches:

1. Change the test project's <Project> tag in the first line to use the web SDK (Microsoft.NET.Sdk.Web instead of Microsoft.NET.Sdk) and add a package reference to Microsoft.AspNetCore.App (or .All if you are using that inside the web project) without specifying a version

   The project file (.csproj) of the test project should now look like this:

   <Project Sdk="Microsoft.NET.Sdk.Web">
   
     <PropertyGroup>
       <TargetFramework>netcoreapp2.1</TargetFramework>
     </PropertyGroup>
   
     <ItemGroup>
       <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="2.1.1" />
       <PackageReference Include="Microsoft.AspNetCore.App" />
       <!-- other references to xunit, test SDK etc. -->
     </ItemGroup>
   
     <ItemGroup>
       <ProjectReference Include="..\AspNetCoreAppToTest\AspNetCoreAppToTest.csproj" />
     </ItemGroup>
   
   </Project>
   

2. Alternative: Leave the Sdk as-is and add a PackageReference to the shared framework package but specify a version.

   This can be done by simply adding a NuGet reference to Microsoft.AspNetCore.App. However, this may cause issues since the SDK may choose to update the reference when a new patch release of the ASP.NET Core is released and the tooling is updated to reflect this. You will need update the NuGet reference for every patch release.

The question specifies ASP.NET Core 2.1, but for those running into the same problem with ASP.NET Core 2.2 it's enough to add the following to your test project file:

<PackageReference Include="Microsoft.AspNetCore.App" />

There is no need to set Sdk="Microsoft.NET.Sdk.Web" anymore and you should not add a version to the package reference above - that causes a warning.