test(complete): Add test for external subcommand

Author: shannmu

clap_complete/tests/testsuite/engine.rs

@@ -981,6 +981,24 @@ a_pos,c_pos"
     );
 }
 
+#[test]
+fn suggest_external_subcommand() {
+    let mut cmd = Command::new("dynamic")
+        .arg(clap::Arg::new("positional").value_parser(["pos1", "pos2", "pos3"]));
+
+    assert_data_eq!(
+        complete!(cmd, " [TAB]"),
+        snapbox::str![
+            "--help\tPrint help
+-h\tPrint help
+pos1
+pos2
+pos3
+"
+        ]
+    );
+}
+
 fn complete(cmd: &mut Command, args: impl AsRef<str>, current_dir: Option<&Path>) -> String {
     let input = args.as_ref();
     let mut args = vec![std::ffi::OsString::from(cmd.get_name())];