As mandated by the Standard's various synopses, e.g. [iterator.synopsis].
Searching the TeX source for '#include' is a good way to find all of these mandates.
The new tests are all autogenerated by utils/generate_header_inclusion_tests.py.
I was SHOCKED by how many mandates there are, and how many of them libc++ wasn't conforming with.
The <iterator> --> <concepts> edge is TODO'ed because that's D99041 / D99044.
All the other TODO'ed edges are because one or the other header doesn't exist in libc++ at all, yet (<ranges>, <coroutine>, <syncstream>).