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 outside shadow trees
2. Shadow host with a positive tabindex
3. Focusable element inside a shadow host with a positive tabindex
4.1. Focusable element inside a shadow host with no tabindex
4.2. Shadow host with no tabindex
5. Last sequentially focusable element outside shadow trees
4.1. Focusable element inside a shadow host with no tabindex
4.2. Shadow host with no tabindex
3. Focusable element inside a shadow host with a positive tabindex
2. Shadow host with a positive tabindex
1. First sequentially focusable element outside shadow trees