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/basic/ratelimit.h

@@ -43,4 +43,4 @@ typedef struct RateLimit {
                 _r->begin = 0;                           \
         } while (false)
 
-bool ratelimit_test(RateLimit *r);
+bool ratelimit_below(RateLimit *r);