docs unittest: Add verification of opcodes covered by RAPI

All opcodes which are not yet covered or can't with the current design
(e.g. cluster initialization) are excluded. This test is added to the
docs unittest since here the code already has access to a list of all
RAPI resources and does more checks on them.

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