testutils: enrich InvenioWebTestCase class
testutils: enrich InvenioWebTestCase class

* Enriches helper functions for web tests in order
  to be able to find elements also by id:
   - element_value_test(element_name="", element_id="")
   - choose_selectbox_option_by_label(selectbox_name="", selectbox_id="")
   - choose_selectbox_option_by_index(selectbox_name="", selectbox_id="")
   - choose_selectbox_option_by_value(selectbox_name="", selectbox_id="")
   - fill_textbox(textbox_name="", textbox_id="")
* Enriches the helper function element_value_test()
  in order to be able to get the value of a given element
  that is not a textarea or a textbox.
* Adds new helper function:
   - find_elements_by_class_name_with_timeout()
* Amends web tests as needed because of this enrichment.
  (addresses #798)

tiborsimko/invenio-formatter,inveniosoftware/invenio-formatter,inveniosoftware/invenio-formatter,tiborsimko/invenio-formatter,tiborsimko/invenio-formatter,inveniosoftware/invenio-formatter