MDL-43839 navigation: Added behat step to navigate from navigation/Admin block

This step will help navigate to different parts of moodle
using navigation or adminstration block.
MDL-43839 behat: Other fixes to make clean work properly

- Update the step to follow links inside a treeview
  to work also when JS is disabled.
- Make regions accept header and footer.
- Change some locators refering to non-clean compatible
  regions.