kunit: mock: added the concept of redirect-mockable functions

Similar to the concept of spying in Mockito
(http://static.javadoc.io/org.mockito/mockito-core/2.20.0/org/mockito/Mockito.html#spy-T-),
this adds the concept of redirect-mockable function declarations which
allow a function declared redirect-mockable to be mocked *and* to allow
the mock to redirect to the original function definition.

Change-Id: I36577b240f72a340732ad86bdd6d8444a5546ef3
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>