Make Python tests only check the current device.

In practice testing all connected devices is a pain, since it's
probably each device is running a different build. It would probably
make sense to just move this functionality up into a higher level test
runner (which could just live in main).

Also rename test_devices to test_shell, since it doesn't really test
`adb devices`.

Change-Id: Ie96d3e83b30acfac4e3bcbd9821690c0ad4d2f7e