tests: use configure.ac, not configure.in, with aclocal/automake involved

tests: use configure.ac, not configure.in, with aclocal/automake involved

Do so because future automake and aclocal versions (starting from 1.13)
drop support for 'configure.in' as the name of the Autoconf input file.
Without this patch, the Autoconf testsuite experiences some spurious
failures when run with the development version of aclocal and automake
installed early enough in $PATH.

* tests/torture.at: Rename 'configure.in' to 'configure.ac' throughout.
Remove an obsolete comment about backward-compatibility.

Helped-by: Jim Meyering <jim@meyering.net>
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>