Increase the tolerance in the disaggregation test

Due to the changes in the arrange algorithm in hazardlib (see https://github.com/gem/oq-hazardlib/pull/187) the numbers in the disaggregation QA test are slightly different, so I have raised the tolerance.
Notice that this is only temporary, since the QA tests have been completely rewritten in the disaggregation branch, soon to appear in a separate pull request.