list: test: Test the hlist structure

Add KUnit tests to the hlist linked-list structure which is used by
hashtables. This should give coverage of every function and macro in
list.h, as well as (combined with the KUnit tests for the hash
functions) get very close to having tests for the hashtable structure.

The tests here mirror the existing list tests, and are found in a new
suite titled 'hlist'.

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>