Add basic Plankton DP mux test

USB type C mux DP and USB3 signals. This test switches the mux between
DP and USB3 and checks if USB3 device is detected.

BUG=chrome-os-partner:33362
TEST=manual
  test_that --fast -b host_board dut_ip firmware_TypeCProbeUSB3

Change-Id: I875b6084556ed237f86313b15a7980efa51706e0
Reviewed-on: https://chromium-review.googlesource.com/257946
Reviewed-by: Wai-Hong Tam <waihong@chromium.org>
Commit-Queue: Pin-chih Lin <johnylin@chromium.org>
Tested-by: Pin-chih Lin <johnylin@chromium.org>
3 files changed