tests/vb2_host_flashrom_tests: Fix missing '\0' for strncpy call

Replace strncpy call with snprintf to auto-append the terminating null
character.

BUG=none
TEST=make run2tests
BRANCH=none

Change-Id: I56d86b66e3d4894b353059e3ed21762b79b63058
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/7077335
Commit-Queue: Hsuan Ting Chen <roccochen@chromium.org>
Reviewed-by: Hsuan Ting Chen <roccochen@chromium.org>