actions: list: Add `for_each_action_{const, mutable}()` macros

Accessing all the inner actions of a action list in a loop is a common
access pattern. This commit adds 2 `for_each` macros to iterate over all
elements either using a const or a mutable pointer.

Add a few unit tests for the list action to test these macros.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I9aff0b81e1f782b5d20c3fcb82ee7028da8dd810