Use .NET SDK in two test projects

[dougbu]

Use .NET SDK in NetCore.Test project

• for now, unable to target netcoreapp2.1 in NetCore.Test project
  • Microsoft.Net.Http is not compatible
  • System.Net.Http types have different identities, causing compilation failures
• switch to use dotnet vstest for any .NET Core assemblies (just one currently)
  • can use Xunit target for all .NET Framework assemblies
• make XunitXml.TestLogger available for .NET Core App testing
  • use above test logger in repo-level testing
  • integrate w/ tests that use the Xunit target
• don't import from xunit.msbuild.runner when targeting .NET Core; not needed
• add $(CopyLocalLockFileAssemblies) to these (now both .NET SDK) test projects
  • xUnit assemblies were not copied (when they should have been) w/o this

Use .NET SDK in Microsoft.TestCommon

• allows reference in .NET SDK test projects w/o duplicating package references
• not sure how NetStandard.Test targeted netcoreapp2.1 before

[dougbu]

@wtgodbe and other reviewers, suggest having another look because I needed to change a fair amount to get things working end-to-end.