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/LdapServer.pm

@@ -169,6 +169,7 @@ sub new
 
 		pidfile $slapd_pidfile
 		logfile $slapd_logfile
+		logfile-only on
 
 		access to *
 		        by * read