C#: Add struct example to test for cs/missing-readonly-modifier.

Author: michaelnebel

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

@@ -8,6 +8,7 @@ class MissedReadonlyOpportunity<T>
     public int Good4;
     public readonly T Good5;
     public T Good6;
+    public Mutable Good7;
 
     public MissedReadonlyOpportunity(int i, T t)
     {
@@ -18,6 +19,7 @@ public MissedReadonlyOpportunity(int i, T t)
         Good4 = i;
         Good5 = t;
         Good6 = t;
+        Good7 = new Mutable();
     }
 
     public void M(int i)
@@ -27,3 +29,13 @@ public void M(int i)
         x.Good6 = false;
     }
 }
+
+struct Mutable
+{
+    private int x;
+    public int Mutate()
+    {
+        x = x + 1;
+        return x;
+    }
+}

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

@@ -1,3 +1,7 @@
+#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:11:20:11:24 | Good7 | Field 'Good7' can be 'readonly'. |
 | MissedReadonlyOpportunityBad.cs:3:9:3:13 | Field | Field 'Field' can be 'readonly'. |
+testFailures
+| MissedReadonlyOpportunity.cs:11:20:11:24 | Field 'Good7' can be 'readonly'. | Unexpected result: Alert |