[otbn] Run OTBN ISS testing in CI
[otbn] Run OTBN ISS testing in CI

This test doesn't do all that much at the moment, but should grow as
we nail down spec issues, and it would be good to make sure nothing
regresses.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

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