test(gateway): keep the recheck timer out of the eunit process (#3304)

fluxer_gateway/src/guild/guild_availability.erl

@@ -269,8 +269,16 @@ schedule_availability_recheck_available_no_timer_test() ->
 
 schedule_availability_recheck_unavailable_sets_timer_test() ->
     State = #{data => #{<<"guild">> => #{<<"features">> => [<<"UNAVAILABLE_FOR_EVERYONE">>]}}},
-    Result = schedule_availability_recheck(State),
-    ?assertEqual(State, Result).
+    Parent = self(),
+    {Pid, MonitorRef} = spawn_monitor(fun() ->
+        Parent ! {scheduled, self(), schedule_availability_recheck(State)}
+    end),
+    receive
+        {scheduled, Pid, Result} -> ?assertEqual(State, Result)
+    end,
+    receive
+        {'DOWN', MonitorRef, process, Pid, normal} -> ok
+    end.
 
 handle_recheck_transition_clears_cache_on_recovery_test() ->
     GuildId = 99001,

fluxer_gateway/src/guild/guild_subscription_handler.erl

@@ -777,7 +777,7 @@ reference_update_member_subscriptions(SessionId, MemberIds, State) ->
 
 drain_mailbox(Acc) ->
     receive
-        Msg -> drain_mailbox([Msg | Acc])
+        {'$gen_cast', _} = Msg -> drain_mailbox([Msg | Acc])
     after 50 -> lists:reverse(Acc)
     end.