Instead of writing a query to psql's stdin, use -c.  This avoids a
failure where psql exits before we write, seen a few times on the build
farm.  Thanks to Tom Lane for the suggestion.

Back-patch to 11, where the LDAP tests arrived.
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/CA%2BhUKGLFmW%2BHQYPeKiwSp5sdFFHtFViCpw4Mh6yAgEx74r5-Cw%40mail.gmail.com