Be more careful about port selection in src/test/ldap/.

Don't just assume that the next port is free; it might not be, or
if we're really unlucky it might even be out of the TCP range.
Do it honestly with two get_free_port() calls instead.

This is surely a pretty low-probability problem, but I think it
explains a buildfarm failure seen today, so let's fix it.

Back-patch to v11 where this script was added.

Discussion: https://postgr.es/m/25124.1568052346@sss.pgh.pa.us

src/test/ldap/t/001_auth.pl

@@ -56,7 +56,7 @@ my $slapd_logfile = "${TestLib::log_path}/slapd.log";
 my $ldap_conf     = "${TestLib::tmp_check}/ldap.conf";
 my $ldap_server   = 'localhost';
 my $ldap_port     = get_free_port();
-my $ldaps_port    = $ldap_port + 1;
+my $ldaps_port    = get_free_port();
 my $ldap_url      = "ldap://$ldap_server:$ldap_port";
 my $ldaps_url     = "ldaps://$ldap_server:$ldaps_port";
 my $ldap_basedn   = 'dc=example,dc=net';