C#: Include NewtonSoft.Json stubs into the flow summaries test.

Author: michaelnebel

csharp/ql/test/library-tests/dataflow/library/options

@@ -1,3 +1,3 @@
 semmle-extractor-options: /nostdlib /noconfig
-semmle-extractor-options: --load-sources-from-project:${testdir}/../../../resources/stubs/_frameworks/Microsoft.NETCore.App/Microsoft.NETCore.App.csproj
+semmle-extractor-options: --load-sources-from-project:${testdir}/../../../resources/stubs/Newtonsoft.Json/13.0.1/Newtonsoft.Json.csproj
 semmle-extractor-options: ${testdir}/../../../resources/stubs/System.Web.cs