Test calls to CDB_Invalidate_Varnish

Also drops the DELETE trigger on CDB_TableMetadata as the call to
CDB_Invalidate_Varnish would then get an OID, which is pretty useless.
Closes #8

sql/test_setup.sql

@@ -1,3 +1,9 @@
 CREATE EXTENSION postgis;
 CREATE EXTENSION schema_triggers;
 CREATE EXTENSION cartodb;
+CREATE FUNCTION public.cdb_invalidate_varnish(table_name text)
+RETURNS void AS $$
+BEGIN
+  RAISE NOTICE 'cdb_invalidate_varnish(%) called', table_name;
+END;
+$$ LANGUAGE 'plpgsql';