[clang-tools-extra] [clang-doc] create a separate cmake file for clang-doc's lit tests (PR #165935)

[Paul Kirth via cfe-commits]

ilovepi wrote:

> I think having a separate unittest target is also perfectly reasonable to do in a follow up patch.

Right. I'm not suggesting its a requirement to land. just an observation that we probably should.

https://github.com/llvm/llvm-project/pull/165935