cp: move option check to uumain and use show_usage_error

- add test for conflicting options `--backup` and `--no-clobber`

src/uu/cp/src/cp.rs

@@ -463,6 +463,12 @@ pub fn uumain(args: impl uucore::Args) -> i32 {
         .get_matches_from(args);
 
     let options = crash_if_err!(EXIT_ERR, Options::from_matches(&matches));
+
+    if options.overwrite == OverwriteMode::NoClobber && options.backup != BackupMode::NoBackup {
+        show_usage_error!("options --backup and --no-clobber are mutually exclusive");
+        return 1;
+    }
+
     let paths: Vec<String> = matches
         .values_of(OPT_PATHS)
         .map(|v| v.map(ToString::to_string).collect())
@@ -593,17 +599,6 @@ impl Options {
 
         let overwrite = OverwriteMode::from_matches(matches);
 
-        if overwrite == OverwriteMode::NoClobber && backup_mode != BackupMode::NoBackup {
-            show_error!(
-                "options --backup and --no-clobber are mutually exclusive\n\
-                 Try '{} --help' for more information.",
-                executable!()
-            );
-            return Err(Error::Error(
-                "options --backup and --no-clobber are mutually exclusive".to_owned(),
-            ));
-        }
-
         // Parse target directory options
         let no_target_dir = matches.is_present(OPT_NO_TARGET_DIRECTORY);
         let target_dir = matches

tests/by-util/test_cp.rs

@@ -539,6 +539,18 @@ fn test_cp_backup_off() {
     assert!(!at.file_exists(&format!("{}~", TEST_HOW_ARE_YOU_SOURCE)));
 }
 
+#[test]
+fn test_cp_backup_no_clobber_conflicting_options() {
+    let (_, mut ucmd) = at_and_ucmd!();
+
+    ucmd.arg("--backup")
+        .arg("--no-clobber")
+        .arg(TEST_HELLO_WORLD_SOURCE)
+        .arg(TEST_HOW_ARE_YOU_SOURCE)
+        .fails()
+        .stderr_is("cp: options --backup and --no-clobber are mutually exclusive\nTry 'cp --help' for more information.");
+}
+
 #[test]
 fn test_cp_deref_conflicting_options() {
     new_ucmd!()