Replace use of stat()[7] by -s switch in TAP tests to retrieve file size

Replace use of stat()[7] by -s switch in TAP tests to retrieve file size

The list form of stat() is an inelegant API as it relies on the position
of the file size in the list returned in result.  Like in any other
places of the tree, replace that with a -s switch instead.

Another suggestion from Dagfinn is File::Stat, which we've been already
using for some other fields.  It really comes down to a matter of taste
to choose that over -s, and the latter is more used in the tree.

Author: Bertrand Drouvot
Reviewed-by: Dagfinn Ilmari Mannsåker
Discussion: https://postgr.es/m/b2020df7-d0fc-4ea5-b2a9-7efc6d36b2ac@gmail.com