Add a 'does_not_support_stepdowns' tag to the geo_borders.js test

geo_borders.js first calls createIndex expecting it to fail, then secondly calls createIndex with a different spec expecting it to succeed. However, both requests have the same default index name, and due to the test suite stepping down the primary frequently, the second request can target a node that hasn't yet aborted the first request.

Simple solution is to add the does_not_support_stepdowns tag to the test. This also protects the subsequent queries expecting to use the index against targeting a primary node with a still incomplete index.