C#: Add property test case where the field keyword is used.

csharp/ql/test/library-tests/properties/PrintAst.expected

@@ -230,3 +230,19 @@ properties.cs:
 #-----|         2: (Parameters)
 #  124|           0: [Parameter] value
 #  124|         4: [BlockStmt] {...}
+#  128|   10: [Class] UseFieldKeyword
+#  130|     6: [Property] Prop
+#  130|       -1: [TypeMention] object
+#  132|       3: [Getter] get_Prop
+#  132|         4: [BlockStmt] {...}
+#  132|           0: [ReturnStmt] return ...;
+#  132|             0: [FieldAccess] access to field Prop.field
+#  133|       4: [Setter] set_Prop
+#-----|         2: (Parameters)
+#  133|           0: [Parameter] value
+#  133|         4: [BlockStmt] {...}
+#  133|           0: [ExprStmt] ...;
+#  133|             0: [AssignExpr] ... = ...
+#  133|               0: [FieldAccess] access to field Prop.field
+#  133|               1: [ParameterAccess] access to parameter value
+#  130|     7: [Field] Prop.field

csharp/ql/test/library-tests/properties/Properties17.expected

@@ -1,3 +1,4 @@
+| Prop.field |
 | caption |
 | next |
 | y |

csharp/ql/test/library-tests/properties/Properties17.ql

@@ -1,5 +1,5 @@
 /**
- * @name Test that there are no backing fields
+ * @name Test that there are no backing fields except for properties that use the `field` keyword in their getter or setter.
  */
 
 import csharp

csharp/ql/test/library-tests/properties/properties.cs

@@ -124,4 +124,13 @@ namespace Properties
             set { }
         }
     }
+
+    class UseFieldKeyword
+    {
+        public object Prop
+        {
+            get { return field; }
+            set { field = value; }
+        }
+    }
 }