In discussing the Windows aspects of the bytes/text boundary handling issues with Brett & Steve recently, I realised I hadn't clearly defined what "fixed" looked like from my perspective.

The attached test case is an initial attempt at that. It currently fails on a UTF-8 Linux system, with the "test_dash_c_unicode" case failing when the interpreter is misconfigured with "LANG=C" - the problem there is that when we encode from the -c command line argument back to bytes, we don't pass "surrogateescape".

I'd be interested in knowing how much of this already passes on a Windows system.

There's also a currently missing test case, which is to pass the info to the subprocess via stdin - "assert_python_ok()" doesn't currently support that, so implementing it will either require a new flag, or direct invocation of spawn_python().