zephyr test: SYV682x: Fix test ordering assumption
zephyr test: SYV682x: Fix test ordering assumption

test_syv682x_vbus_enable checked "initial" state of PPC emulator based
on no-longer-valid assumptions about test ordering. Test VBUS source
disabling in an order-independent way in a separate test. Clarify test
names.

BUG=b:234886404
TEST=zmake testall; verify coverage did not change
BRANCH=none

Signed-off-by: Abe Levkoy <2e1ad7039298f93262cf06bbb457275fb3e2ecb5@chromium.org>
Change-Id: Id3e43bf6ef38d5c93cedfd8dfc6dfe69c9e63137
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3690269
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>

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