LDAP check flapping on crake due to race

Hi,

There are one or two failures per month on crake.  It looks like when
authentication is rejected, as expected in the tests, the psql process
is exiting, but there is a race where the Perl script still wants to
write a dummy query to its stdin (?), so you get:

psql: FATAL:  LDAP authentication failed for user "test1"
ack Broken pipe: write( 13, 'SELECT 1' ) at
/usr/share/perl5/vendor_perl/IPC/Run/IO.pm line 549.

Example:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=crake&dt=2019-11-10%2023%3A36%3A04

tmunro=> select animal, snapshot, branch from run where fail_stage =
'ldapCheck' order by snapshot desc;
 animal |      snapshot       |    branch
--------+---------------------+---------------
 crake  | 2020-08-02 02:32:30 | REL_13_STABLE
 crake  | 2020-07-22 23:36:04 | REL_12_STABLE
 crake  | 2020-07-14 00:52:04 | REL_13_STABLE
 crake  | 2020-05-15 17:35:05 | REL_11_STABLE
 crake  | 2020-04-07 20:51:03 | REL_12_STABLE
 mantid | 2020-03-04 18:17:58 | REL_12_STABLE
 mantid | 2020-03-04 17:59:58 | REL_11_STABLE
 crake  | 2020-01-17 14:33:21 | REL_12_STABLE
 crake  | 2019-11-10 23:36:04 | REL_11_STABLE
 crake  | 2019-09-09 08:48:25 | HEAD
 crake  | 2019-08-05 21:18:23 | REL_12_STABLE
 crake  | 2019-07-19 01:33:31 | HEAD
 crake  | 2019-07-16 01:06:02 | REL_11_STABLE
(13 rows)

(Ignore mantid, it had a temporary setup problem that was resolved.)