kunit: executor_test: Use kunit_add_action()

Now we have the kunit_add_action() function, we can use it to implement
kfree_at_end() and free_subsuite_at_end() without the need for extra
helper functions.

Reviewed-by: Benjamin Berg <benjamin.berg@intel.com>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>