tests: add test for polytope topology

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: add function thorium_node_get_exporter_worker()

This function is used to find out which worker is the exporter
worker for given node.

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>