Move routines to manipulate WAL into PostgreSQL::Test::Cluster

 Move routines to manipulate WAL into PostgreSQL::Test::Cluster
 
 These facilities were originally in the recovery TAP test
 039_end_of_wal.pl.  A follow-up bug fix with a TAP test doing similar
 WAL manipulations requires them, and all these had better not be
 duplicated due to their complexity.  The routine names are tweaked to
 use "wal" more consistently, similarly to the existing "advance_wal".
 
 In v14 and v13, the new routines are moved to PostgresNode.pm.
 039_end_of_wal.pl is updated to use the refactored routines, without
 changing its coverage.
 
 Reviewed-by: Alexander Kukushkin
 Discussion: https://postgr.es/m/CAFh8B=mozC+e1wGJq0H=0O65goZju+6ab5AU7DEWCSUA2OtwDg@mail.gmail.com
 Backpatch-through: 13