nmigen_cfu/cfu.py: allow ignore expected
nmigen_cfu/cfu.py: allow ignore expected

Allows Instruction test cases to ignore results. With this change,
InstructionTestBase matches the functionality of CfuTestBase.

Signed-off-by: Alan Green <97e77871b5fcc4941bebceb4338c97c1fd149f8c@google.com>

google/CFU-Playground,google/CFU-Playground,google/CFU-Playground,google/CFU-Playground