Remove hardcoded versions from runtests.proj.
Put project.json for Xunit test wrappers in file system instead of generating. This lets dependency validation check the versions and reduces the number of project.json files to restore because all csproj's point to a single project.json and lockfile.
Adds XunitPackageVersion property that is used to always generate a valid project.json for xunit.runner.msbuild.
Commit migrated from https://github.com/dotnet/coreclr/commit/
351c4c4c7a32197343501ffa466752b344af5539