fpsensor: Improve unit test for enable/disable positive match secret
fpsensor: Improve unit test for enable/disable positive match secret

Initialize the local struct so that the test is not flaky. Also
extract helper function and add more asserts to make the logic more
explicit to readers.

BUG=none
BRANCH=nocturne
TEST=make -j buildall

Change-Id: I84ccd097b64c3ff304f4af228121b2cb989e6ab7
Signed-off-by: Yicheng Li <77a205905d8955597c069555e9569c30512f6bd8@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1865956
Reviewed-by: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>

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