tests: Make sure we look in the environment for MACHINE variable.
tests: Make sure we look in the environment for MACHINE variable.

Fixes a test breakage on Beaglebone, where this variable is only
defined in the environment.

Changelog: None

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@mender.io>

bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender