osmo_hexparse: allow whitespace in parsed string, add ws test

This is particularly useful for hex dumps containing spaces found in a log
(e.g. osmo-nitb authentication rand token), which can now be passed in quotes
to osmo-auc-gen without having to edit the spaces away.

Change-Id: Ib7af07f674a2d26c8569acdee98835fb3e626c45
3 files changed