List available behat data generators on admin/tool/behat/index.php

Currently the step definitions reference on admin/tools/behat/index.php lists the available step definitions, and the available options for some of the fields in a definition (for example, '"ELEMENT_STRING" "SELECTOR_STRING" should exist' displays the available selectors in place of SELECTOR_STRING).

When using a data generator via the 'the following "ELEMENT_STRING" exist:' step, the only ways to find out what elements are available are by searching for other uses of the step in a feature file, or by looking at the code in behat_core_generator::get_creatable_entities() and others.

It would make writing tests a lot easier of the step definitions list displayed a similar select list with the entities that can be generated.