Add cli-deps feed for dotnet-test-xunit

Although there is a dedicated `coreclr-xunit` feed for this package, it
is missing some needed dependencies. Rather than add yet another feed
(`aspnetvnext`), we'll use the `cli-deps` feed as it has all the
packages that `dotnet-test-xunit` needs, and is likely to stick around.

nuget.config

@@ -3,6 +3,7 @@
   <packageSources>
     <clear />
     <add key="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="nuget.org" value="https://api.nuget.org/v3/index.json" />
     <add key="local" value="./src/windows-build/nuget-feed" />
   </packageSources>