rapi.testutils: Add exported functions to verify opcode input/result

These can be used by third-party code to verify mock code. Further work
on mocks is forthcoming, so this is only a start.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>