squash! stbt batch report: Use fixed table-layout (TODO: Test layout on other browsers & OSs)

I can't use `max-width` (instead of `width`) for the Timestamp &
Duration columns; if the window is wide enough those columns will get
bigger than the specified `max-width`. The CSS 2.1 spec† seems to
suggest that only `width` is valid for `table-layout:fixed` columns:

> In the fixed table layout algorithm, the width of each column is
> determined as follows:
>
> 1. A column element **with a value other than 'auto' for the 'width'
> property** sets the width for that column.
> 2. Otherwise, a cell in the first row **with a value other than 'auto'
> for the 'width' property** determines the width for that column. If
> the cell spans more than one column, the width is divided over the
> columns.
> 3. Any remaining columns equally divide the remaining horizontal table
> space (minus borders or cell spacing).

(emphasis mine)

† http://www.w3.org/TR/CSS21/tables.html#fixed-table-layout