Add explicit test for wrong OpCode arbitrary instances

I've been bitten a couple of times with arbitrary opcodes working on
UTF-8 locale, but failing on buildbot (ASCII). So let's add an
explicit test that checks always (even with UTF-8) for correct
arbitrary values, showing explicitly which opcodes fail.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>