Fix flakiness in limitsortingtbl test

[morgando]

This test checks that an expensive sorter query fails when a tunable is set to block sorter queries that push the DB past a disk usage limit.

The limit’s set to 1% higher than current usage, so the idea is that running an expensive query will go over the limit and get rejected.

This PR just makes the query heavier so it’s more likely to hit that limit consistently.

[morgando]

/adhoc-test limitsortingtbl