C#: Add ref field test case.

csharp/ql/test/library-tests/csharp11/PrintAst.expected

@@ -612,6 +612,24 @@ Strings.cs:
 #   53|           1: [StringLiteralUtf8] "The nested message
 #   53|             is UTF-8 encoded and
 #   53|           spans multiple lines."
+Struct.cs:
+#    1| [NamespaceDeclaration] namespace ... { ... }
+#    3|   1: [Class] MyEmptyClass
+#    5|   2: [Struct] RefStruct
+#    7|     5: [Field] MyInt
+#    7|       -1: [TypeMention] int
+#    8|     6: [Field] MyByte
+#    8|       -1: [TypeMention] byte
+#    9|     7: [Field] MyObject
+#    9|       -1: [TypeMention] object
+#   10|     8: [Field] MyEmptyClass
+#   10|       -1: [TypeMention] MyEmptyClass
+#   11|     9: [Field] MyReadonlyByte
+#   11|       -1: [TypeMention] byte
+#   12|     10: [Field] MyReadonlyObject
+#   12|       -1: [TypeMention] object
+#   13|     11: [Field] MyReadonlyString
+#   13|       -1: [TypeMention] string
 cil/class1.cs:
 #    4| [Class] Class1
 #    6|   5: [Method] Main

csharp/ql/test/library-tests/csharp11/Struct.cs

@@ -1,5 +1,4 @@
-// TODO: Test needs to be enabled when .NET 7 is used as the runtime.
-namespace structassembly;
+namespace structassembly;
 
 public class MyEmptyClass { }
 

csharp/ql/test/library-tests/csharp11/refField.expected

@@ -1,3 +1,11 @@
 reffields
+| Struct.cs:8:21:8:26 | MyByte |
+| Struct.cs:9:23:9:30 | MyObject |
+| Struct.cs:10:31:10:42 | MyEmptyClass |
+| Struct.cs:12:32:12:47 | MyReadonlyObject |
 readonlyreffields
+| Struct.cs:11:30:11:43 | MyReadonlyByte |
+| Struct.cs:13:41:13:56 | MyReadonlyString |
 readonlyfield
+| Struct.cs:12:32:12:47 | MyReadonlyObject |
+| Struct.cs:13:41:13:56 | MyReadonlyString |