sys: lists: Add tests for newly-added list len functions

This commit adds tests for the newly added list len APIs to the
corresponding test cases. It is noted that the test functions calculate
equivalent values manually using several different internal list
functions. This has been left unmodified to ensure that the manual ways
using each of the various for_each functions results in the same value
as the new list_len() functions.

Signed-off-by: J M <montytyper@msn.com>