test-serve: disable unfixable tests on Windows
These tests would run if hghave.has_serve() were enabled on Windows. Windows
has no issue allowing an unpriviledged process to open port 13, so it doesn't
abort. The other tests are related to how MSYS tries to be helpful and converts
Unix constructs to the Windows equivalent. There isn't any way to disable this
behavior, though it supposedly doesn't happen if the exe is linked against the
MSYS library.