Recreate the parallel WAL-restore coverage in the plugin repo: a 2-instance
cluster archiving to minio with wal.maxParallel=3, forged WAL segments on the
object store, and assertions on the plugin's prefetch/spool/end-of-wal-stream
state machine driven through `/controller/manager wal-restore` on the standby.
Part of cloudnative-pg/cloudnative-pg#10954.
Signed-off-by: Armando Ruocco <armando.ruocco@enterprisedb.com>