fix(test): IO safety violations while dropping the fd of open file

[WeepingClown13]

With stdlib changes File now uses OwnedFd internally that does IO safety checks in debug mode and that consequently causes the test here to panic as FDs of open files are closed behind stdlib's back. So this patch uses the low level libc::open to directly return a RawFd, skipping the std::fs::File::open calls and as_raw_fd conversion.

Fixes: #1