Move from dotnet.config to global.json (#3553)

* Delete dotnet.config
* Update global.json

dotnet.config

@@ -1,2 +0,0 @@
-[dotnet.test.runner]
-name = "Microsoft.Testing.Platform"

global.json

@@ -3,5 +3,8 @@
     "version": "10.0.0",
     "rollForward": "major",
     "allowPrerelease": true
+  },
+  "test": {
+    "runner": "Microsoft.Testing.Platform"
   }
-}
+}