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)