Updating tests to use official xunit
* Reacting to System.Threading version change
* Removing System.Threading.ExecutionContext since it no longer exists
Merge pull request #989 from aspnet/xunit-update

Updating tests to use official xunit
Made changes to how the load context works
- Load from default load context before custom loaders in the
AssemblyLoadContext.
- At runtime, the IAssemblyLoadContextFactory delegates to the default load
context.
- Use the IAssemblyLoadContextFactory to create a child load context
and load the project loader in that. This should make it work at build time
and within design time host.
- Removed the IAssemblyNeutralInterfaceCache
- Load ANIs in the default load context
Update klr.core45 to use minimal TPA list

This is a temporary change.
Made more changes related to load contexts for preprocessing
- Create a child load context within roslyn itself. This
is required since it relies on the project loader being in place.
[Draft] Build minimal set of runtime required assembly
Fix the missing var in makefile.shade
Update BuildTpaCommand, skip project type library
[Draft] prototype of the tpa list
Update the tpa list build logic
Try native images first during TPA build
Update tpa.cpp in dpa
Update tpa.cpp during build
Avoid rewrite the tpa.cpp if not necessary
Review feedback #1

1. Remove whitespace in declarations
2. Revert non-essential fomatting in klr.core45.cpp
Update coreclr copy task
Use the LibraryManager property on ApplicationHostContext directly
Stop looking into assemblies from 'Project' type library
Review feedback #2