C#: Add the EntityFramework stub to the general flow summaries test.

Author: michaelnebel

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

@@ -2,3 +2,4 @@ semmle-extractor-options: /nostdlib /noconfig
 semmle-extractor-options: --load-sources-from-project:${testdir}/../../../resources/stubs/Newtonsoft.Json/13.0.1/Newtonsoft.Json.csproj
 semmle-extractor-options: --load-sources-from-project:../../../resources/stubs/_frameworks/Microsoft.AspNetCore.App/Microsoft.AspNetCore.App.csproj
 semmle-extractor-options: ${testdir}/../../../resources/stubs/System.Web.cs
+semmle-extractor-options: ${testdir}/../../../resources/stubs/EntityFramework.cs