test: Make default screen timeout depend on the VALGRIND env var

While running under valgrind, the screen can take significantly longer to
update(especially on travis) so a higher timeout can be required. Also reduce
the timeout when not running on valgrind.

test/functional/ui/screen.lua

@@ -85,6 +85,10 @@ Screen.__index = Screen
 
 local debug_screen
 
+local default_screen_timeout = 2500
+if os.getenv('VALGRIND') then
+  default_screen_timeout = 7500
+end
 
 function Screen.debug(command)
   if not command then
@@ -187,7 +191,7 @@ function Screen:wait(check, timeout)
     end
     return true
   end
-  run(nil, notification_cb, nil, timeout or 5000)
+  run(nil, notification_cb, nil, timeout or default_screen_timeout)
   if not checked then
     err = check()
   end