list: test: Add a test for list_entry_is_head()

The list_entry_is_head() macro was added[1] after the list KUnit tests,
so wasn't tested. Add a new KUnit test to complete the set.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e130816164e244b692921de49771eeb28205152d



Signed-off-by: David Gow <davidgow@google.com>
Acked-by: Daniel Latypov <dlatypov@google.com>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>