kunit: mock: add class mocking support

Introduce 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.

Co-developed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>