std: Ignore a test on windows

This test is known to fail on windows.

src/libstd/fs/mod.rs

@@ -1552,6 +1552,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg(not(windows))]
     fn unlink_readonly() {
         let tmpdir = tmpdir();
         let path = tmpdir.join("file");