Add e2e for header keyboard navigation

[alizedebray]

Since the header and megadropdown keyboard navigation is controlled via JavaScript, it would be helpful to have end-to-end tests to ensure we are not introducing regressions when touching these components.

Test cases:

• desktop
  • the close button is the last tab element
  • clicking tab after the close button closes the megadropdown and focuses the next nav item
• mobile and tablet
  • with the first level opened, all elements from the global header, local header and burger menu can receive focus
  • with second level opened, only the elements from the megadropdown can receive focus
  • back button is the last element