Fix pgbench TAP test failure with funky file names (redux)

Fix pgbench TAP test failure with funky file names (redux)

This test fails if the containing directory contains a funny character
such as a space or some perl metacharacter. To avoid that, we check for
files names using readdir and a regex, rather than using a glob pattern.

Discussion: https://postgr.es/m/CAM6_UM6dGdU39PKAC24T+HD9ouy0jLN9vH6163K8QEEzr__iZw@mail.gmail.com

Author: Fabien COELHO
Reviewed-by: Raúl Marín Rodríguez