Add missing span in tests

src/Analyzers/CSharp/Tests/MakeFieldReadonly/MakeFieldReadonlyTests.cs

@@ -1743,7 +1743,7 @@ public async Task WithThreadStaticAttribute_NoDiagnostic()
 class Program
 {
     [ThreadStatic]
-    private static object t_obj;
+    private static object [|t_obj|];
 }");
         }
     }

src/Analyzers/VisualBasic/Tests/MakeFieldReadonly/MakeFieldReadonlyTests.vb

@@ -976,7 +976,7 @@ End Class")
 
 Class C
     <ThreadStatic>
-    Private Shared t_obj As Object
+    Private Shared [|t_obj|] As Object
 End Class")
         End Function
     End Class