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/test/test-ratelimit.c

@@ -11,24 +11,24 @@
 #include "ratelimit.h"
 #include "time-util.h"
 
-static void test_ratelimit_test(void) {
+static void test_ratelimit_below(void) {
         int i;
         RATELIMIT_DEFINE(ratelimit, 1 * USEC_PER_SEC, 10);
 
         for (i = 0; i < 10; i++)
-                assert_se(ratelimit_test(&ratelimit));
-        assert_se(!ratelimit_test(&ratelimit));
+                assert_se(ratelimit_below(&ratelimit));
+        assert_se(!ratelimit_below(&ratelimit));
         sleep(1);
         for (i = 0; i < 10; i++)
-                assert_se(ratelimit_test(&ratelimit));
+                assert_se(ratelimit_below(&ratelimit));
 
         RATELIMIT_INIT(ratelimit, 0, 10);
         for (i = 0; i < 10000; i++)
-                assert_se(ratelimit_test(&ratelimit));
+                assert_se(ratelimit_below(&ratelimit));
 }
 
 int main(int argc, char *argv[]) {
-        test_ratelimit_test();
+        test_ratelimit_below();
 
         return 0;
 }