Explicitly list dependencies in TestUtilities project.json, the new nuget.exe...

Explicitly list dependencies in TestUtilities project.json, the new nuget.exe doesn't flow them anymore.

src/Test/Utilities/Portable/project.json

 {
   "dependencies": {
     "Microsoft.NETCore.Portable.Compatibility": "1.0.0",
-    "System.Collections.Concurrent": "4.0.10"
+    "System.Collections.Concurrent": "4.0.10",
+    "System.AppContext": "4.0.0",
+    "System.Collections": "4.0.10",
+    "System.Collections.Immutable": "1.1.37",
+    "System.Console": "4.0.0-beta-23123",
+    "System.Diagnostics.Debug": "4.0.10",
+    "System.Globalization": "4.0.10",
+    "System.IO.FileSystem": "4.0.0",
+    "System.IO.FileSystem.Primitives": "4.0.0",
+    "System.Reflection": "4.0.10",
+    "System.Reflection.Metadata": "1.2.0",
+    "System.Runtime": "4.0.20",
+    "System.Runtime.Extensions": "4.0.10",
+    "System.Runtime.InteropServices": "4.0.20",
+    "System.Text.Encoding": "4.0.10",
+    "System.Threading": "4.0.10",
+    "System.Threading.Tasks": "4.0.10",
+    "System.Threading.Thread": "4.0.0-beta-23123",
+    "System.Xml.XDocument": "4.0.10",
+    "System.Xml.XmlDocument": "4.0.0",
+    "xunit": "2.1.0",
+    "xunit.extensibility.execution": "2.1.0"
   },
   "frameworks": {
     "dotnet": {