[verification] Bump default timeout to 20 seconds.

src/verification.ml

@@ -210,10 +210,11 @@ let create_env ?(debug = false) loadpath =
   let config = Autodetect.autodetection ~debug () in
   let config =
     let main = Whyconf.get_main config in
+    let dft_timelimit = 20 (* 20 seconds *) in
     let dft_memlimit = 4000 (* 4 GB *) in
     let main =
       Whyconf.(
-        set_limits main (timelimit main) dft_memlimit (running_provers_max main))
+        set_limits main dft_timelimit dft_memlimit (running_provers_max main))
     in
     Whyconf.set_main config main
   in