Use clear_npcs in clear_map (#30051)

* Use clear_npcs in clear_map

clear_map had a similar flaw to the one fixed in #30031 affecting
npc_can_target_player.  Fix it in the same way.

This wasn't obviously affecting any tests, but it makes sense to fix it
pre-emptively.

* Before killing npcs, ensure correct ones loaded

Some tests
(default_overmap_generation_has_non_mandatory_specials_at_origin)
overwrite overmaps, which leads to orphaned NPCs.  Unloading and
reloading the NPCs ensures that all the active NPCs are known to the
overmap.

tests/map_helpers.cpp

@@ -35,11 +35,14 @@ void clear_creatures()
 {
     // Remove any interfering monsters.
     g->clear_zombies();
-    g->unload_npcs();
 }
 
 void clear_npcs()
 {
+    // Unload and reaload to ensure that all active NPCs are in the
+    // overmap_buffer.
+    g->unload_npcs();
+    g->load_npcs();
     for( npc &n : g->all_npcs() ) {
         n.die( nullptr );
         overmap_buffer.remove_npc( n.getID() );
@@ -72,6 +75,7 @@ void clear_map()
     }
     wipe_map_terrain();
     g->m.clear_traps();
+    clear_npcs();
     clear_creatures();
 }