Tests for moving focus by pressing tab key across shadow boundaries.
To manually test, press tab key sixteen times then shift+tab sixteen times.
It should traverse focusable elements in the increasing numerical order and then in the reverse order.

1. First sequentially focusable element

A non-focusable shadow host should not be focused.

5. Slotted content with tabindex=0

4. Slotted content with tabindex=4

A non-focusable shadow host should not be focused.

11. Content in slot 2 with tabindex=1

10. Content in slot 2 with tabindex=1

12. Content in slot 2 with tabindex=0

9. Content in slot 1 with tabindex=0

8. Content in slot 1 with tabindex=7

A non-focusable shadow host should not be focused.