Fix test warnings.

Author: sunfishcode

tests/cap-basics.rs

@@ -217,10 +217,11 @@ fn symlink_loop_from_rename() {
     check!(tmpdir.open("link"));
 }
 
-#[cfg(linux)]
+#[cfg(target_os = "linux")]
 #[test]
 fn proc_self_fd() {
-    let fd = check!(File::open("/proc/self/fd"));
+    let fd = check!(std::fs::File::open("/proc/self/fd"));
     let dir = cap_std::fs::Dir::from_std_file(fd);
-    error!(dir.open("0"), "No such file");
+    // This should fail with "too many levels of symbolic links".
+    dir.open("0").unwrap_err();
 }