Add tests verifying behavior when an IntersectionObserver has an
explicit element root:
- A target that is a descendant of the root should be reported as
intersecting when it overlaps the root.
- A target that is NOT a descendant of the root but spatially overlaps
it should NOT be reported as intersecting per spec step 3 of "run
the update intersection observations steps". The expected output
currently reflects the wrong (buggy) behavior.
