autotest: add test network_3GModemControl

Add the test network_3GModemControl, and add it to the network_3g
suite.  This test ensures that one can use either modem, flimflam
technology, or flimflam device commands to control the modem, and in
all cases the modem and flimflam state should stay synchronized.

BUG=chromium-os:21089
TEST=run this test

Change-Id: I2c9178bc3376a5071b460fbe1882fa50f0822b53
Reviewed-on: https://gerrit.chromium.org/gerrit/10809
Commit-Ready: Jason Glasgow <jglasgow@chromium.org>
Reviewed-by: Jason Glasgow <jglasgow@chromium.org>
Tested-by: Jason Glasgow <jglasgow@chromium.org>
4 files changed