BUG (output formatting): use fixed with for truncation column instead of inferring from last column
Fixes https://github.com/pandas-dev/pandas/issues/24849

We were using the last column to infer the width of the truncation column (= the column with all the `...` if not all columns are shown). This gives wrong results in case the last column is very wide, and since the content of this columns is always fixed to `...`, we can also use a fixed width I think.