I can't override the user_pass hook
The main error is that, since your module's short name is beneficiaire, those are the implementations of hook_user_pass_validate() and hook_user_pass_submit(), which aren't hook Drupal utilizes. If you rename the functions beneficiaire_validate() and beneficiaire_submit(), those would not be hooks which are used as validation and submission handlers for the password change form.
See @Clive's answer for how adding form validation and submission handlers, keeping in mind that the validation handler is added with $form['#validate'][] = 'beneficiaire_user_pass_validate';.

As side notes:


If you want to check a validation handler is called, use form_set_error("", "beneficiaire_user_pass_validate() called");
To redirect the user after submitting a form, set $form_state['redirect'] as in $form_state['redirect'] = 'login';