Add basic TAP tests for the low-level backup method

Add basic TAP tests for the low-level backup method

There are currently no tests for the low-level backup method where
pg_backup_start() and pg_backup_stop() are involved while taking a
file-system backup.  The tests introduced in this commit rely on a
background psql process to make sure that the backup is taken while the
session doing the SQL start and stop calls remains alive.

Two cases are checked here with the backup taken:
- Recovery without a backup_label, leading to a corrupted state.
- Recovery with a backup_label, with a consistent state reached.
Both cases cross-check some patterns in the logs generated when running
recovery.

Author: David Steele
Discussion: https://postgr.es/m/f20fcc82-dadb-478d-beb4-1e2ffb0ace76@pgmasters.net