AutoTest: reduce time of copter fly_circle

Circle_Rate parameter was increased so we can reduce the time of the
test

Tools/autotest/arducopter.py

@@ -427,7 +427,7 @@ def fly_simple(mavproxy, mav, side=100, timeout=120):
     return not failed
 
 #fly_circle - flies a circle with 20m radius
-def fly_circle(mavproxy, mav, maxaltchange=10, holdtime=72):
+def fly_circle(mavproxy, mav, maxaltchange=10, holdtime=36):
 
     '''hold loiter position'''
     mavproxy.send('switch 5\n') # loiter mode