vim-patch:9.2.0927: curswant not set on 8g8 (#41255)

Problem:  curswant not set on 8g8
Solution: Set curswant, adjust tests (Emilien Breton)

closes: vim/vim#20979

7fe5cb35f9

Co-authored-by: Emilien Breton <bricktech2000@gmail.com>

src/nvim/mbyte.c

@@ -2135,6 +2135,7 @@ void utf_find_illegal(void)
             curwin->w_cursor.col += l;
           }
         }
+        curwin->w_set_curswant = true;
         goto theend;
       }
       p += len;