Rename ratelimit_test to ratelimit_below

When I see "test", I have to think three times what the return value
means. With "below" this is immediately clear. ratelimit_below(&limit)
sounds almost like English and is imho immediately obvious.

(I also considered ratelimit_ok, but this strongly implies that being under the
limit is somehow better. Most of the times this is true, but then we use the
ratelimit to detect triple-c-a-d, and "ok" doesn't fit so well there.)

C.f. a1bcaa07.

src/core/socket.c

@@ -2245,7 +2245,7 @@ static void socket_enter_running(Socket *s, int cfd) {
                 return;
         }
 
-        if (!ratelimit_test(&s->trigger_limit)) {
+        if (!ratelimit_below(&s->trigger_limit)) {
                 safe_close(cfd);
                 log_unit_warning(UNIT(s), "Trigger limit hit, refusing further activation.");
                 socket_enter_stop_pre(s, SOCKET_FAILURE_TRIGGER_LIMIT_HIT);