zephyr test: Make ppc_discharge_vbus mockable
zephyr test: Make ppc_discharge_vbus mockable

Allow tests to mock ppc_discharge_vbus. Support tests of discharging
behavior that are decoupled from the discharge indicator of a specific
PPC.

BUG=b:255413715
TEST=make buildall
BRANCH=none

Signed-off-by: Abe Levkoy <2e1ad7039298f93262cf06bbb457275fb3e2ecb5@chromium.org>
Change-Id: Ie6678c6fcbb30fa94fc0403071fd6366f794973f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3989344
Reviewed-by: Al Semjonovs <c3a2ecb550cad1e7d5a943ca6241377f7b43ffc4@google.com>

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