test: Clarify the types of variables

[muramura]

I want to reduce the build warnings. I reduced the warnings by making the types explicit.

[AppVeyorBot]

:white_check_mark: Build MissionPlanner 1.0.2641 completed (commit https://github.com/ArduPilot/MissionPlanner/commit/1df47f9dc3 by @muramura)