[OTBN/DIF] Use a non-zero error code in mock tests
[OTBN/DIF] Use a non-zero error code in mock tests

Previously, only an error code with value 0 was available for testing,
now we also have a non-zero error code available. Use a non-zero error
code to also catch cases where zero is the default (uninitialized)
value.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan