software/erp5: fix flappy test_logrotate_and_slow_query_digest test

This test uses the default config of having mariadb log queries taking more
than 1 second, then makes two slow queries and check that these two queries
are logged and included in pt-query-digest report.

Sometimes this test fail because there are more slow queries than the two
expected, in the snapshots, we can see that reports also contain:

    SHOW TABLE STATUS FROM `mysql` LIKE 'time_zone_transition'\G
    SHOW CREATE TABLE `mysql`.`time_zone_transition`\G

but these queries are not made by the test. They are probably made by
mysql_upgrade script. Such queries can take more than 1 second sometimes
and when they do the test fail because there are more slow queries than the
expected.

To prevent this kind of intermitent failures, make our test queries slower
and configure mariadb to log queries after a longer delay (3 seconds).
Hopefully these unrelated queries should not take more than 3 seconds.