[clang-tools-extra] [clang-doc] Try to make testing more uniform (PR #205586)

[Douglas Yung via cfe-commits]

dyung wrote:

Oh I think I see the issue, `clang-test-extra/test/clang-doc/json/long-name.cpp` is marked with `UNSUPPORTED: system-windows`, but the nearly identical test `clang-test-extra/test/clang-doc/html/long-name.cpp` is not. I'll put up a PR to mark the latter as also unsupported on Windows.

https://github.com/llvm/llvm-project/pull/205586