C#: Add test case for sealed ToString modifier on a record type.

Author: michaelnebel

csharp/ql/test/library-tests/csharp10/recordTypeSealedToString.expected

@@ -0,0 +1 @@
+| RecordTypeSealedToString.cs:5:35:5:42 | ToString |

csharp/ql/test/library-tests/csharp10/recordTypeSealedToString.ql

@@ -0,0 +1,3 @@
+import csharp
+
+query predicate sealed(Modifiable m) { m.getDeclaringType().fromSource() and m.isSealed() }