C#: Add a data flow test for side effects on static fields

csharp/ql/test/library-tests/dataflow/fields/K.cs

@@ -0,0 +1,19 @@
+public class K
+{
+    static string[] Strings = new string[10];
+
+    private void M1()
+    {
+        var o = Source<string>(1);
+        Strings[0] = o;
+    }
+
+    private void M2()
+    {
+        Sink(Strings[0]); // $ MISSING: hasValueFlow=1
+    }
+
+    public static void Sink(object o) { }
+
+    static T Source<T>(object source) => throw null;
+}