Add low-level cell locate test.
Fix requirement in WiFi lookups to really only require two networks.

Our clustering algorithm accidentally discarded clusters of two or less
networks, so you still required three networks for lookups.
Change argument order for search_all_sources.
Already round the return values of search_all_sources.
Move some detailed tests over to new test_locate module.
Avoid duplicate stats testing in test_locate and use a neutral metric name.