Skip write_error test if running as root (#1881)

tests/fmt.rs

@@ -103,6 +103,13 @@ fn unstable_passed() {
 
 #[test]
 fn write_error() {
+  // skip this test if running as root, since root can write files even if
+  // permissions would otherwise forbid it
+  #[cfg(not(windows))]
+  if unsafe { libc::getuid() } == 0 {
+    return;
+  }
+
   let tempdir = temptree! {
     justfile: "x    :=    'hello'   ",
   };