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/timesync/timesyncd-manager.c

@@ -804,7 +804,7 @@ int manager_connect(Manager *m) {
         manager_disconnect(m);
 
         m->event_retry = sd_event_source_unref(m->event_retry);
-        if (!ratelimit_test(&m->ratelimit)) {
+        if (!ratelimit_below(&m->ratelimit)) {
                 log_debug("Slowing down attempts to contact servers.");
 
                 r = sd_event_add_time(m->event, &m->event_retry, clock_boottime_or_monotonic(), now(clock_boottime_or_monotonic()) + RETRY_USEC, 0, manager_retry_connect, m);