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.

nuget.config

@@ -2,8 +2,10 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="dotnet-core" value="https://www.myget.org/F/dotnet-core/api/v3/index.json" />
+    <add key="CI Builds (dotnet-core)" value="https://www.myget.org/F/dotnet-core/api/v3/index.json" />
-    <add key="cli-deps" value="https://dotnet.myget.org/F/cli-deps/api/v3/index.json" />
+    <add key="CI Builds (dotnet-cli)" value="https://dotnet.myget.org/F/dotnet-cli/api/v3/index.json" />
+    <add key="CI Builds (aspnetvnext)" value="https://www.myget.org/F/aspnetvnext/api/v3/index.json" />
+    <add key="CI Builds (aspnetcidev)" value="http://myget.org/F/aspnetcidev/api/v3/index.json" />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
     <add key="local" value="./src/windows-build/nuget-feed" />
   </packageSources>

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"
             }
         }
     },