Upgrading test-related libraries

NUnit 3.4.0 —> 3.6.0
dotnet-test-nunit 3.4.0-alpha-2 —> 3.4.0-beta-3

csharp/src/Google.Protobuf.Test/project.json

@@ -20,8 +20,8 @@
 
   "dependencies": {
     "Google.Protobuf": { "target": "project" },
-    "NUnit": "3.4.0",
-    "dotnet-test-nunit": "3.4.0-alpha-2"
+    "dotnet-test-nunit": "3.4.0-beta-3",
+    "NUnit": "3.6.0"
   },
 
   "testRunner": "nunit",