sys: add truncate option to OpenOptions in test case

this resolves a clippy lint that checks for uses of `create()` without
`truncate()` [1].

[1]:
https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_open_options

Signed-off-by: Shannon Sterz <s.sterz@proxmox.com>

proxmox-sys/tests/xattr.rs

@@ -14,6 +14,7 @@ fn test_fsetxattr_fgetxattr() {
     let file = OpenOptions::new()
         .write(true)
         .create(true)
+        .truncate(true)
         .open(&path)
         .unwrap();