perl tap: Show failed command output

perl tap: Show failed command output

Capture stdout and stderr from command_ok() and command_fails() and emit
them as TAP diagnostics on failure.  Output is truncated to the first
and last 30 lines per channel to avoid flooding.

A new helper _diag_command_output() is introduced in Utils.pm so
both functions share the same truncation and formatting logic.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/DFYFWM053WHS.10K8ZPJ605UFK@jeltef.nl