as for failure, an unknown answer just before the time limit should be considered as a timeout

this allows to provide the time limit on the command of CVC4, and perhaps Z3 later.

src/driver/call_provers.ml

@@ -154,7 +154,7 @@ let parse_prover_run res_parser time out ret on_timelimit timelimit =
   Debug.dprintf debug "Call_provers: prover output:@\n%s@." out;
   let time,steps = Opt.get_def (time,-1) (grep_time out res_parser.prp_timeregexps) in
   let ans = match ans with
-    | HighFailure when on_timelimit && timelimit > 0
+    | Unknown _ | HighFailure when on_timelimit && timelimit > 0
       && time >= (0.9 *. float timelimit) -> Timeout
     | _ -> ans
   in