Upgrade Test project dependencies and dotnet build improvement

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Minor upgrades and improvements to test projects.

1. Introduce Microsoft.NETFramework.ReferenceAssemblies to metadatagenerator for portability of dotnet build across platforms. Upgrade dependencies for tools projects.
2. Upgrade dependencies for test projects. (Excluding CodeAnalysis)
3. Ignore xunit extension project as part of dotnet test command.

Fixes build portability with metadatagenerator

dotnet build

C:\Program Files\dotnet\sdk\3.1.100\Microsoft.Common.CurrentVersion.targets(1175,5): 
error MSB3644: The reference assemblies for .NETFramework,Version=v4.8 were not found. 
To resolve this, install the Developer Pack (SDK/Targeting Pack) for 
this framework version or retarget your application. 
You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks 

As part of running metadatagenerator to verify documentation, I notice some are out of date and may need updates.