Add license headers on all files

tests/by-util/test_kill.rs

@@ -1,3 +1,7 @@
+// This file is part of the uutils coreutils package.
+//
+// For the full copyright and license information, please view the LICENSE
+// file that was distributed with this source code.
 use crate::common::util::TestScenario;
 use regex::Regex;
 use std::os::unix::process::ExitStatusExt;