kunit: mock: added basic mock infrastructure

Introduces basic class mocking, the ability to automatically generate a
Linux C-style class implementation whose behavior is controlled by test
cases, which can also set expectations on when and how mocks are called.

Change-Id: Id9d535be1da95dd42844c43f6dfcca3a7441bd81
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>