Enable Nominatim mock in OptaPlanner 8.13

OptaWeb Vehicle Routing has an end-to-end test that uses the geosearch feature (looking up coordinates for a given address). The app uses https://nominatim.openstreetmap.org, which is a public online service. The public server often has downtimes and since the mentioned test depends on it, the CI stability suffers. Fortunately, it's easy to mock the geosearch API and return the same cached response without contacting the service during the test.

Let's backport this to the 8.13.x branch for a smooth 7.13 release.