autotest: we need more than 1500 throttle for SimpleMode test

we are in stabilize and flying around, needs more than half throttle
to maintain height. This test was already marginal, but addition of
pressure alt in SITL pushed it over the edge

Tools/autotest/arducopter.py

@@ -1438,7 +1438,7 @@ class AutoTestCopter(AutoTest):
         # switch to stabilize mode
         self.mavproxy.send('switch 6\n')
         self.wait_mode('STABILIZE')
-        self.set_rc(3, 1500)
+        self.set_rc(3, 1545)
 
         # fly south 50m
         self.progress("# Flying south %u meters" % side)