test: Add test case for completing custom value of argument

Author: shannmu

clap_complete/tests/testsuite/dynamic.rs

@@ -590,6 +590,13 @@ val3
     );
 }
 
+#[test]
+fn suggest_custom_arg_value() {
+    let mut cmd = Command::new("dynamic").arg(clap::Arg::new("custom").long("custom"));
+
+    assert_data_eq!(complete!(cmd, "--custom [TAB]"), snapbox::str![""],);
+}
+
 #[test]
 fn suggest_multi_positional() {
     let mut cmd = Command::new("dynamic")