16791: Fixes audit log cleaning tests.

16791: Fixes audit log cleaning tests.

Log fixtures lacking the created_at field set in the past in combination
with incorrect testing made us believe the LogTest was working ok.
Specifically, LogTest#test_delete_old_audit_logs_in_thread was not doing
the waiting for the log entries to be eliminated, so there was a race
between the AuditLogs thread cleaning the records and the test teardown doing
the rollback producing the intermittent failure of
use_ownership_and_permission_links_to_determine_which_logs_a_user_can_see.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <lucas@di-pentima.com.ar>