tests: Minimize syslog activity by slapd

Until now the tests using slapd spammed syslog for every connection /
query. Use logfile-only to prevent syslog activity. Unfortunately that only
takes effect after logging the first message, but that's still much better
than the prior situation.

Discussion: https://postgr.es/m/20230311233708.3yjdbjkly2q4gq2j@awork3.anarazel.de
Backpatch: 11-

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

@@ -90,6 +90,7 @@ include $ldap_schema_dir/inetorgperson.schema
 
 pidfile $slapd_pidfile
 logfile $slapd_logfile
+logfile-only on
 
 access to *
         by * read