Move admin0 server functions around

- move files to its right place
- add some boilerplate for test setup
- add tests

server/extension/sql/30_admin0_test.sql

@@ -0,0 +1,17 @@
+-- Check that the public function is callable, even with no data
+-- It should return NULL
+SELECT cdb_geocoder_server.geocode_admin0_polygons(session_user, txid_current(), 'Spain');
+
+-- Insert some dummy synonym
+INSERT INTO admin0_synonyms (name, adm0_a3) VALUES ('Spain', 'ESP');
+
+-- Insert some dummy geometry to return
+INSERT INTO ne_admin0_v3 (adm0_a3, the_geom) VALUES('ESP', ST_GeomFromText(
+  'POLYGON((-71.1031880899493 42.3152774590236,
+            -71.1031627617667 42.3152960829043,
+            -71.102923838298 42.3149156848307,
+            -71.1031880899493 42.3152774590236))',4326)
+);
+
+-- This should return the polygon inserted above
+SELECT cdb_geocoder_server.geocode_admin0_polygons(session_user, txid_current(), 'Spain');