Refactor common code out from network_3GSmokeTest for reuse.

Also clean up several issues reported by pylint.

BUG=chromium:238859
TEST=Ran the following tests:
- network_3GSmokeTest/control  # with Gobi3k and E362
- network_3GSmokeTest/control.pseudomodem

Change-Id: I65905cc3a0f9e665637320aac6acad1b66f1e252
Reviewed-on: https://gerrit.chromium.org/gerrit/50379
Reviewed-by: Thieu Le <thieule@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
2 files changed