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-