Fix xUnit tests for Linux

The latest xUnit packages fix the "could not resolve coreclr path"
problem we were having. To resolve all dependencies, the cli-deps feed
was replaced with the aspnet feeds.

However, the latest xUnit packages do not allow us to set the default
AssemblyLoadContext.

test/csharp/project.json

@@ -12,9 +12,8 @@
         "netstandardapp1.5": {
             "imports": [ "dnxcore50", "portable-net45+win8" ],
             "dependencies": {
-                "NuGet.Packaging.Core": "3.5.0-beta-1145",
                 "xunit": "2.1.0",
-                "dotnet-test-xunit": "1.0.0-dev-91790-12"
+                "dotnet-test-xunit": "1.0.0-dev-140469-38"
             }
         }
     },