Test preadv and pwritev on more operating systems

This was an oversight from PR #1511

Author: asomers

test/sys/test_uio.rs

@@ -141,7 +141,7 @@ fn test_pread() {
 }
 
 #[test]
-#[cfg(target_os = "linux")]
+#[cfg(not(target_os = "redox"))]
 fn test_pwritev() {
     use std::io::Read;
 
@@ -171,7 +171,7 @@ fn test_pwritev() {
 }
 
 #[test]
-#[cfg(target_os = "linux")]
+#[cfg(not(target_os = "redox"))]
 fn test_preadv() {
     use std::io::Write;