C#: Add test for missing reverse aliasing flow

Author: hvitved

csharp/ql/test/library-tests/dataflow/reverse-flow/ReverseFlow.cs

@@ -0,0 +1,64 @@
+public class A
+{
+    public string Field;
+
+    public A Nested;
+
+    public void M1()
+    {
+        var a = new A();
+        M2(a);
+        Sink(a.Nested.Field); // $ MISSING: hasValueFlow=1
+    }
+
+    public void M2(A a)
+    {
+        var b = a.Nested;
+        M3(b);
+    }
+
+    public void M3(A a)
+    {
+        a.Field = Source<string>(1);
+    }
+
+    public void M4()
+    {
+        this.M5();
+        Sink(this.Nested.Field); // $ MISSING: hasValueFlow=1
+    }
+
+    public void M5()
+    {
+        var b = this.Nested;
+        b.M6();
+    }
+
+    public void M6()
+    {
+        this.Field = Source<string>(2);
+    }
+
+    public void M7()
+    {
+        var a = new A();
+        M8(a);
+        Sink(a.Field); // $ MISSING: hasValueFlow=3
+    }
+
+    public void M8(A a)
+    {
+        var b = new A();
+        b.Nested = a;
+        M9(b);
+    }
+
+    public void M9(A a)
+    {
+        a.Nested.Field = Source<string>(3);
+    }
+
+    public static void Sink(object o) { }
+
+    static T Source<T>(object source) => throw null;
+}

csharp/ql/test/library-tests/dataflow/reverse-flow/ReverseFlow.expected

@@ -0,0 +1,6 @@
+models
+edges
+nodes
+subpaths
+testFailures
+#select

csharp/ql/test/library-tests/dataflow/reverse-flow/ReverseFlow.ql

@@ -0,0 +1,12 @@
+/**
+ * @kind path-problem
+ */
+
+import csharp
+import TestUtilities.InlineFlowTest
+import DefaultFlowTest
+import PathGraph
+
+from PathNode source, PathNode sink
+where flowPath(source, sink)
+select sink, source, sink, "$@", source, source.toString()