Fix TestPruneLogsBySize
Fix TestPruneLogsBySize

This would occasionally fail because the logs would be pruned down to
below 5000 (since it was slightly above the threshold before
pruning). Instead check that there are more than 2000 records - this
should never be hit. The thing that I was actually concerned with when
adding the check was that there should still be *some* records left in
the collection (that was a bug I introduced while making changes).

mjs/juju,mjs/juju,mjs/juju,mjs/juju,mjs/juju,mjs/juju,mjs/juju