C#: Add cs/missed-readonly-modifier test for assigning private field in argument of same type.

Author: michaelnebel

csharp/ql/test/query-tests/Language Abuse/MissedReadonlyOpportunity/MissedReadonlyOpportunity.cs

@@ -39,3 +39,24 @@ public int Mutate()
         return x;
     }
 }
+
+class Tree
+{
+    private Tree? Parent;
+    private Tree? Left; // $ Alert
+    private readonly Tree? Right;
+
+    public Tree(Tree left, Tree right)
+    {
+        this.Left = left;
+        this.Right = right;
+        left.Parent = this;
+        right.Parent = this;
+    }
+
+    public Tree()
+    {
+        Left = null;
+        Right = null;
+    }
+}

csharp/ql/test/query-tests/Language Abuse/MissedReadonlyOpportunity/MissedReadonlyOpportunity.expected

@@ -1,3 +1,8 @@
+#select
 | MissedReadonlyOpportunity.cs:3:16:3:19 | Bad1 | Field 'Bad1' can be 'readonly'. |
 | MissedReadonlyOpportunity.cs:4:14:4:17 | Bad2 | Field 'Bad2' can be 'readonly'. |
+| MissedReadonlyOpportunity.cs:45:19:45:24 | Parent | Field 'Parent' can be 'readonly'. |
+| MissedReadonlyOpportunity.cs:46:19:46:22 | Left | Field 'Left' can be 'readonly'. |
 | MissedReadonlyOpportunityBad.cs:3:9:3:13 | Field | Field 'Field' can be 'readonly'. |
+testFailures
+| MissedReadonlyOpportunity.cs:45:19:45:24 | Field 'Parent' can be 'readonly'. | Unexpected result: Alert |