Set explicit version for Microsoft.AspNetCore.App in Test Project to fix build errors

[Franklin89]

This PR should fix the Build Errors you were having on the last stream after setting an implicit version.

There are two workarounds at the moment:

1. Change the test project's 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
2. Leave the Sdk as-is and add a PackageReference to the shared framework package but specify a version.

This PR uses the second workaround.

@natemcmaster if you have time could you elaborate on the reason why exactly it is required and what the future plan is?

More details and discussions:

dotnet/aspnetcore#3307

https://stackoverflow.com/questions/50401152/integration-and-unit-tests-no-longer-work-on-asp-net-core-2-1-failing-to-find-as/50401153#50401153

[natemcmaster]

if you have time could you elaborate on the reason why exactly it is required and what the future plan is?

Sure, happy to clarify. It is required for the following reasons:

• Referencing a shared framework does not flow across ProjectReference dotnet/sdk#2420 - the Microsoft.AspNetCore.App package uses an MSBuild property to tell the .NET Core SDK which shared framework is used. In MSBuild, properties from packages to not flow transitively across ProjectReference, so you must have a PackageReference in each project (see also Allow package authors to define build assets which work transitively (using buildTransitive assets group) NuGet/Home#6091)
• The versionless PackageReference only works with Microsoft.NET.Sdk.Web in the 2.1 SDK. This has been fixed in the 2.2 SDK. The original design of this feature did not account for usage of Microsoft.AspNetCore.App outside of Microsoft.NET.Sdk.Web. But in the 2.2 SDK update, the versionless PackageReference will work in console, test, and web projects for both netcoreapp2.1 and netcoreapp2.2. See Add support in .NET SDK for implicit versions of AspNetCore.App and .All dotnet/sdk#2533

Future plan:

• Replace PackageReference to Microsoft.AspNetCore.App with FrameworkReference dotnet/aspnetcore#3612 - in netcoreapp3.0, there will no longer be a PackageReference at all. We're completely refactoring the way this metapackage works. (See also https://github.com/aspnet/AspNetCore/labels/area-shared-framework)

[Franklin89]

@natemcmaster thanks a lot 👍

[csharpfritz]

Thank you @Franklin89 for fixing this... I now feel more confident in the build results from Azure Build Pipelines