Enable project restore for test projects (dotnet/corefx#38151)

Enable project restore for test projects (dotnet/corefx#38151)

* Factor out repo restore into seperate props/targets

* Use configuration system msbuild sdk

* Remove XUnit.Runtime.depproj

* Remove TargetGroup usages in test projects

* Fix ILLink configuration

The ILLink package contains netcoreapp2.0 assets which are compatible
with netstandard2.0. Removing the old TargetFramework to restore
successfully.

* Remove usages from TargetGroup in Directory.Build.props

* Remove dead net4* configurations

* Change tests.builds to tests.proj sdk proj

* Make externals dir proj an sdk project

* Consume Microsoft.DotNet.CoreFxTesting as an msbuild sdk

* Use runtimeconfig.template.json

* Remove SupplementalTestData

* Map shared framework version to product version

* Stop binplacing tests

* Move some tool depprojs to targets during restore

* Remove dead code

* Use PackageReference instead of RefFromRuntime in test

* Disable using an apphost

* Copy compression testdata manually cause unicode bug

* Remove target batching in optimizationData

* Reference packaging tools from CoreFx.Private.TestUtilities

Commit migrated from https://github.com/dotnet/corefx/commit/c4c1985531cc1f25c805bbc4801b85c98e0dfee0