Add more TAP tests with BASE_BACKUP and pg_backup_start/stop

Add more TAP tests with BASE_BACKUP and pg_backup_start/stop

This commit adds some test coverage for ee79647 (prevent BASE_BACKUP
from running in the middle of another base backup) and b24b2be
(BASE_BACKUP cancellation followed by pg_backup_start), caused by the
interactions of replication and SQL commands in a logical replication
connection in a WAL sender.

The second test uses a design close to what has been introduced in
0475a97f, where BASE_BACKUP is throttled to give enough room for a
cancellation, though this time we rely on psql with multiple -c
switches to keep a connection around for the second query.

Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/Ys/NCI4Eo9300GnQ@paquier.xyz