Update VS install on CI machines to a version with the bundled .NET Core / .NET Standard .props, and re-enable tests

[dsplaisted]

In #1052, I'm removing the RuntimeFrameworkVersion from test projects that target .NET Core 2.0. This doesn't work with the version of VS installed on the CI machines yet, so I'm disabling the following tests on full framework:

• Microsoft.NET.Build.Tests.GivenThatWeWantToReferenceAnAssembly.ItRunsAppsDirectlyReferencingAssemblies
• Microsoft.NET.Build.Tests.GivenThatWeWantToVerifyProjectReferenceCompat.Project_reference_compat (when targeting .NET Core 2.0)
• Microsoft.NET.Build.Tests.GivenThatWeWantToVerifyNuGetReferenceCompat.Nuget_reference_compat (when targeting .NET Core 2.0 or .NET Standard 2.0)
• Microsoft.NET.Publish.Tests.GivenThatWeWantToPublishAHelloWorldProject.Conflicts_are_resolved_when_publishing_a_portable_app
• Microsoft.NET.Publish.Tests.GivenThatWeWantToPublishAHelloWorldProject.Conflicts_are_resolved_when_publishing_a_self_contained_app
• Microsoft.NET.Publish.Tests.GivenThatWeWantToPublishAHelloWorldProject.Publish_standalone_post_netcoreapp2_app_and_it_should_run
• Microsoft.NET.Publish.Tests.GivenThatWeWantToPublishAHelloWorldProject.Conflicts_are_resolved_when_publishing_a_portable_app

We need to update the CI machines to a newer version of VS and then re-enable these tests.

See also #1041, we will probably fix both of these together.