test(complete): Ensure we infer path completions from value_parser
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-rs/clap,clap-rs/clap,clap-rs/clap