autotest: fixed marginal test WPNAV_SPEED_DN

this test completed at 59s before the baro change, and needs 62s
afterwards with the slight change in atmospheric modelling

increase to 70s so it doesn't flap

Author: tridge

Tools/autotest/arducopter.py

@@ -3780,7 +3780,7 @@ def WPNAV_SPEED_DN(self):
 
         minimum_duration = 5
 
-        self.takeoff(500, timeout=60)
+        self.takeoff(500, timeout=70)
         self.change_mode('AUTO')
 
         start_speed_ms = self.get_parameter('WPNAV_SPEED_DN') / 100.0