Fix LDAP test instability.

After starting slapd, wait until it can accept a connection before
beginning the real test work.  This avoids occasional test failures.
Back-patch to 11, where the LDAP tests arrived.

Author: Thomas Munro
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/20190719033013.GI1859%40paquier.xyz