C#: Base the modelgenerator/dataflow tests on stubs.

Author: michaelnebel

csharp/ql/test/utils/modelgenerator/dataflow/options

@@ -1,2 +1,3 @@
-semmle-extractor-options: /r:System.Linq.dll /r:System.Collections.Specialized.dll
+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: ${testdir}/../../../resources/stubs/System.Web.cs