Update testProject nuget packages and targetframework

[eoehen]

Update nuget packages of the testproject:

• Microsoft.NET.Test.Sdk from 15.5.0 to 16.8.3
• xUnit from 2.3.1 to 2.4.1

TargetFramework from netcoreapp2.0 to netcoreapp 2.2
DotNetCliToolReference dotnet-xunit removed

[gep13]

@eoehen thank you for taking the time to raise this PR. Was there a specific reason that you were looking to have these changes included?

We are currently working on the 2.1.0 release of Magic Chunks, so this change likely won't be included in that release, but we may consider it after that release is completed.

[pascalberger]

@eoehen Thanks for your contribution. There's one thing which additionally needs to be updated manually.

[eoehen]

@gep13 Sorry for the misunderstanding. I ask @pascalberger first, if I can create a PR for updating this packages. You can use it for a later version.

[pascalberger]

LGTM

[pascalberger]

Build is currently failing: https://ci.appveyor.com/project/sergeyzwezdin/magic-chunks/builds/37326190

[eoehen]

@pascalberger Build is fixed.

[pascalberger]

LGTM