[otbn,dv] Allow unknown instructions in otbn_env_cov
[otbn,dv] Allow unknown instructions in otbn_env_cov

Without this, UVM tests will fail if coverage is turned on and they
see an unknown instruction. This wasn't a problem before because we
didn't generate any bad instructions, but the next commit will add
"bad instruction support" to the RIG.

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

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