autotest: fix race condition in button test

The subsequent test could end up arming the vehicle based on this left-over state.

Tools/autotest/common.py

@@ -10445,12 +10445,14 @@ switch value'''
             self.progress("Skipping arm/disarm tests for tracker")
             return
 
+        self.context_push()
         self.wait_ready_to_arm()
         self.set_parameter("BTN_FUNC%u" % btn, 153)  # ARM/DISARM
         self.set_parameter("SIM_PIN_MASK", mask)
         self.wait_armed()
         self.set_parameter("SIM_PIN_MASK", 0)
         self.wait_disarmed()
+        self.context_pop()
 
         if self.is_rover():
             self.context_push()