cellular: Migrate network_3GRebootStress to new test_environment

Also fixed incorrect USB device path for non-Gobi modems.

BUG=chromium:415293
TEST=Run network_3GRebootStress on e362, gobi3k, hefty

Change-Id: I2956015a79dfd20552c7852314f214786db10bfc
Reviewed-on: https://chromium-review.googlesource.com/222895
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Thieu Le <thieule@chromium.org>
Tested-by: Thieu Le <thieule@chromium.org>
Commit-Queue: Thieu Le <thieule@chromium.org>
3 files changed