It seems that you are still trying to run these tests in a dotnet core preview runtime but that you tried to update it with a chocolatey command (which is now commented out). The 2.1.0rc-1 release package was added to chocolatey just today. Have you tried uncommenting that line (i think choco upgrade dotnetcore --pre would perhaps be preferable)?
as my image is Visual Studio Preview I thought it will come with the SDK but not sure how often appVeyor update the VS preview image and not even if it really comes with the SDK (I updated to the latest preview and anyway had to install the sdk manually)
Yes, you are correct but, alas, sometimes image updates don't happen perfectly on schedule (particularly when conferences get in the way). You can follow the main platform updates here. Once Visual studio is updated on the main vs2017 image, you can expect that the preview will then refer to the 15.8 preview.
Also, you can see whats upcoming in the 'next-images-update' milestone.