The code (prior to d25825a754) assumed
that level 3+ headings would always follow a level 2 heading or the
placeholder heading, but we don't generate a placeholder heading if
there are no comments in section zero.
Add more tests to confirm that comments under level 3+ headings (that
are not sub-headings of level 2), and level 1 headings, are ignored
when generating notifications, and do not mess with normal headings.
Bug: T288775
Change-Id: Ic57b56752a4797cb01234f66e0ed7b849752bd70
