Fix JitBench (dotnet/coreclr#19584)

Fix JitBench (dotnet/coreclr#19584)

Fix JitBench

Fixes https://github.com/dotnet/coreclr/issues/19569
- I believe the issue was that JitBench (which was an netstandard1.6 project) was running against a netcoreapp3.0 layout with corerun (layout built by build + recent packages), and XmlDocument is in a different place now
- Copied the unofficial project and replaced the official one
- Switched to netcoreapp3.0 along with other miscellaneous fixes to get it working
- I haven't figured out yet how to get one project that builds with the test build and works with dotnet. TargetFramework would have to be different anyway if running with dotnet runtime 2.1. So the unofficial project is still there for use with dotnet.
- Also retargeted the empty console project and changed to always set ExePath (when using existing setup for instance)

Commit migrated from https://github.com/dotnet/coreclr/commit/98c34c45f9ad3ec93bf4192735621341506f0550