Tests for setting a negative tabindex on shadow host. Elements inside such a shadow tree should not be in the sequential navigation order.
To manually test, press tab key four times. It should traverse focusable elements in the increasing numerical order.

1. First sequentially focusable element
2. An iframe without a focusable element
3. First focusable element inside an iframe
4. Second focusable element inside an iframe
5. An iframe without a focusable shadow host or shadow content
6. Focusable content inside a shadow tree
7. A focusable element with a high tabindex
8. Focusable content inside the first shadow tree
9. Focusable content inside the second shadow tree
8. Focusable content inside the first shadow tree
7. A focusable element with a high tabindex
6. Focusable content inside a shadow tree
5. An iframe without a focusable shadow host or shadow content
4. Second focusable element inside an iframe
3. First focusable element inside an iframe
2. An iframe without a focusable element
1. First sequentially focusable element