Say why the watchdog exits the ordinary way

It calls exit and not _exit, and the reason is the opposite of what the
comment said: the rows that did pass are still in stdout's buffer, and a
watchdog that threw them away would tell you less than the hang did.
Also observed firing from test_dev, which is blocked on a daemon rather
than spinning — the case the reader hang does not cover.

test/watchdog.ml

@@ -39,6 +39,8 @@ let dying _ =
     \      turned into a failing run.\n"
     !label !budget;
   flush stderr;
+  (* [exit] rather than [_exit]: the rows that did pass are in stdout's buffer
+     and a watchdog that threw them away would be worse than the hang. *)
   exit 2
 
 (* Re-arm the backstop for whatever is left of its budget. One second is the