Generalize the OpCode-should-be-in-mcpu test

Currently, the unittest TestDispatchTable in mcpu unittest does a
hard-coded approach to test whether an opcode should be included or
not in the mcpu.Processor dispatch table. This is not flexible, so we
replace it with two changes:

- first, we do not return the base OpCode in opcodes.OP_MAPPING;
  rationale being that we shouldn't need to serialise or deserialise
  this opcode during normal operation
- second, we add a new attribute WITH_LU (other name suggestions
  welcome) to the base opcode set to True, and we explicitly set it to
  False in OpTestDummy, thus automatic skipping of such LUs in the
  unittest (and in other places)

To correct for the new behaviour, the tests in the opcode unittests
are updated to include OpCode explicitly.

Note: we also replace assert_ with assertTrue in the affected unittest
(as assert_ is deprecated in 2.7).

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