test-hbitmap: Add non-advancing iter_next tests

Add a function that wraps hbitmap_iter_next() and always calls it in
non-advancing mode first, and in advancing mode next.  The result should
always be the same.

By using this function everywhere we called hbitmap_iter_next() before,
we should get good test coverage for non-advancing hbitmap_iter_next().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20180613181823.13618-9-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>