Makes two changes to our groovy arguments

1. Prefer - over / which will let us remove our hacky argument change
legacy script.
1. Use the positive -testDesktop over -skipCoreClr verbs

The Microbuild leg now almost completely builds.  It's failing in the
validation of our compilers Nuget package.  This comes from subtle
changes to Toolset.csproj that need to be addressed.

This does represent a huge step forward though.  Saving this now so I
can experiment with that change.

This is an automated migration of our project files to the new SDK /
PackageReference.  It is done using the tool located here:

https://github.com/jaredpar/ConvertPackageRef

This updates our script for generating and verifying the compiler syntax trees,
bound trees and error facts.

The complication here is we needed to move to fully runnable .NET Core
applications.  There was a lot of hacks / bad data laying around from
our previous hacky .NET Core app build story.  Now that we are on SDK we
can remove that and just do the right thing.

This changes our package verification logic to run inside of BuildBoss
instead of RepoUtil.  The verification is really easy now that package
references are 100% MSBuild based.  No reason to do this in RepoUtil
anymore.

As a result this also fixes most of the correctness leg.  Still need to
fix up the generators as they aren't deploying as EXEs anymore.  Need to
understand how to fix that.

The content produced by NuGet restore is independent of build
configuration.  Hence it needs to be stored in a folder that is
accessible to both Debug and Release.

This didn't work with the present structure of our Obj directory
because it put project folders under configuration directories.  That
mean there is no common parent for a project across all configurations.
To fix this we need to change our Obj structure:

- Old: `Obj\<Configuration>\<Project Name>`
- New: `Obj\<Project Name>\<Configuration>`

This changes our NuGet download and commits us to package reference
elements.

Create the Packages.props and FixedPackages.props.  These are the files
which will contain our MSBuild mapping from packag name to package
version

regression test for  https://github.com/dotnet/roslyn/issues/19119

enabled one test for a fixed bug.

Fix #18905 - No error reported for an expression bodied accessor in abstract event (changes after review)

The trait NetCore was being unconditionally passed as the trait option to our unit tests.  That mean all tests were being filtered
to those which had the trait NetCore which is a very small number.  Fixiyng it to only be passed when running the net core suites

Break was inserted via bad copy / paste in the following

https://github.com/dotnet/roslyn/commit/970cf1762ed422df17a7a22646be0d178d1c139f

Upgrade Roslyn to consume newer Microsoft.Net.RoslynDiagnostics

Disabling flaky tests failed in last 24hr.