Original code relied on POSIX feature. This change doesn't completely enable process_launch tests on Windows, but it's another step in the chain.
I think this should be _MSC_VER. mingw toolchains will still have <unistd.h> and getcwd. We compile all our tests with clang, which does define _MSC_VER when in MSABI compat mode. lgtm otherwise, I'll commit later.