Add a test to verify that install can be used just to copy a simple file

example:
touch bar.txt && ./target/debug/coreutils install bar.txt foo.txt

tests/by-util/test_install.rs

@@ -31,6 +31,19 @@ fn test_install_basic() {
     assert!(at.file_exists(&format!("{}/{}", dir, file2)));
 }
 
+#[test]
+fn test_install_copy_file() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let file1 = "test_install_target_dir_file_a1";
+    let file2 = "test_install_target_dir_file_a2";
+
+    at.touch(file1);
+    ucmd.arg(file1).arg(file2).succeeds().no_stderr();
+
+    assert!(at.file_exists(file1));
+    assert!(at.file_exists(file2));
+}
+
 #[test]
 fn test_install_failing_not_dir() {
     let (at, mut ucmd) = at_and_ucmd!();