Fix recovery test failing in PostgreSQL 12.0.
Fix recovery test failing in PostgreSQL 12.0.

This test was not creating recovery.signal when testing with --type=preserve.  The preserve recovery type only keeps existing files and does not create any.

RC1 was just ignoring recovery.signal and going right into recovery.  Weirdly, 12.0 used restore_command to do crash recovery which made the problem harder to diagnose, but this has now been fixed in PostgreSQL and should be released in 12.1.

pgmasters/backrest,pgbackrest/pgbackrest,pgbackrest/pgbackrest,pgbackrest/pgbackrest,pgmasters/backrest