Test: Add TC unit test for source disconnect detection
Test: Add TC unit test for source disconnect detection

Add a TC layer test for source disconnect, and also add some
infrastructure to detect the test board's Vbus output.

BRANCH=None
BUG=b:153071799
TEST=make -j buildall

Signed-off-by: Diana Z <cb2016afc7183e5356c690dc63b4ba817470501a@chromium.org>
Change-Id: I72f2553d59b939bf236e0ae3ea6248df899ef4ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2921283
Reviewed-by: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec