test_wwrite_to_fd: remove redundant assertion

Also should have been -1, not 0.
Ref: https://github.com/fish-shell/fish-shell/pull/12199#discussion_r2677040787

src/wutil/mod.rs

@@ -660,7 +660,6 @@ fn test_wwrite_to_fd() {
         for &size in &sizes {
             let fd = unsafe {
                 let res = libc::open(filename.as_ptr(), O_RDWR | O_TRUNC | O_CREAT, 0o666);
-                assert!(res != 0);
                 OwnedFd::from_raw_fd(res)
             };
             let mut input = Vec::new();