C#: Add test that shows unintended flow summary generation

csharp/ql/test/utils/modelgenerator/dataflow/CaptureContentSummaryModels.expected

@@ -1,2 +1,3 @@
 unexpectedModel
+| Unexpected contentbased-summary found: Models;HigherOrderParameters;false;Apply;(System.Func<System.Object,System.Object>,System.Object);;Argument[1];ReturnValue;value;dfc-generated |
 expectedModel

csharp/ql/test/utils/modelgenerator/dataflow/Summaries.cs

@@ -536,6 +536,12 @@ public class HigherOrderParameters
     {
         a(o);
     }
+
+    private void CallApply()
+    {
+        // Test that this call to `Apply` does not interfere with the flow summaries generated for `Apply`
+        Apply(x => x, null);
+    }
 }
 
 public static class HigherOrderExtensionMethods