[llvm] [llvm] Run headers-winhttp.test only if the Python side of it works (PR #187727)

[Stefan Gränitz via llvm-commits]

weliveindetail wrote:

For reference, the affected bots are: [llvm-clang-win-x-aarch64](https://lab.llvm.org/buildbot/#/builders/193), [llvm-clang-x86_64-sie-win](https://lab.llvm.org/buildbot/#/builders/46), [llvm-nvptx-nvidia-win](https://lab.llvm.org/buildbot/#/builders/54), [llvm-nvptx64-nvidia-win](https://lab.llvm.org/buildbot/#/builders/155)

https://github.com/llvm/llvm-project/pull/187727