Add helper scripts for testing the shell completions

[bash]

While working on #2896 I needed a way to test the shell completions.

So I put together a script for each shell (PowerShell is notably missing) that spawns the shell
with the right PATH / completions setup. It might be useful to other people too :)

[keith-hall]

Hi, thanks for this. I'm keen to improve the developer experience for testing changes made to completions files, to help encourage us all to do a better job of keeping them up to date. (re: #3170)
Now that we have #3126, do you think any changes need to be made to these test helper scripts?
I had the idea that it might be nice to create a subfolder and put them all together there to reduce clutter, but maybe it's not necessary as they all have the same name just different extension 🤔

[epage]

btw https://docs.rs/completest-pty/latest/completest_pty/ can be used to write tests runnable in cargo test for completions. Its what clap_complete is using (to a limited degree; its slow)