Add basic TAP tests for the low-level backup method, take two

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.

Compared to the first attempt in 99b4a63bef94, this includes a couple of
fixes making the CI stable (5 runs succeeded here):
- Add the file to the list of tests in meson.build.
- Race condition with the first WAL segment that we expect in the
primary's archives, by adding a poll on pg_stat_archiver.  The second
segment with the checkpoint record is archived thanks to pg_backup_stop
waiting for it.
- Fix failure of test where the backup_label does not exist.  The
cluster inherits the configuration of the first node; it was attempting
to store segments in the first node's archives, triggering failures with
copy on Windows.
- Fix failure of test on Windows because of incorrect parsing of the
backup_file in the success case.  The data of the backup_label file is
retrieved from the output pg_backup_stop() from a BackgroundPsql written
directly to the backup's data folder.  This would include CRLFs (\r\n),
causing the startup process to fail at the beginning of recovery when
parsing the backup_label because only LFs (\n) are allowed.

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

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