std::rt::io::support: Fix ignored test on Win32

Assumes drive C: exists.

Closes #8812.
std::os: Use unicode for last_os_error() on Win32

FormatMessageA may return non-ascii message,
which is encoded as system code page, not utf8.
This may cause `assert!(is_utf8(v))` failure on
some non-English machines.
This patch replaces it with FormatMessageW,
which returns utf-16 message.

Fixes `make check-stage2-std` failure on my machine. :)
std::rt::uv::uvll: Fix uv_req_type on Win32

Also enables request_sanity_check() test.

Closes #8817