test(complete): Ensure we infer path completions from value_parser

Looks like I forgot to add a test case for this.

I put this in `clap_complete` because I expect `ValueHint` to move here
as we move towards a plugin system.

Fixes #3840

clap_complete/tests/general.rs

@@ -0,0 +1,11 @@
+#[test]
+fn infer_value_hint_for_path_buf() {
+    let mut cmd = clap::Command::new("completer")
+        .arg(clap::Arg::new("input").value_parser(clap::value_parser!(std::path::PathBuf)));
+    cmd.build();
+    let input = cmd
+        .get_arguments()
+        .find(|arg| arg.get_id() == "input")
+        .unwrap();
+    assert_eq!(input.get_value_hint(), clap::builder::ValueHint::AnyPath);
+}