Create separate nu-testbin binary

[IanManske]

Description

This PR removes the --testbin flag on the main nushell binary and moves this functionality to the new nu-testbin binary. This binary was added to the workspace and marked as publish = false in its Cargo.toml. After this, we will no longer ship testing code to our users, and the removal of the --testbin flag will make the CLI slightly less cluttered.

User-Facing Changes

"Breaking" change: removes the --testbin flag from the main nushell binary.

[IanManske]

Still some open questions:

• Where should the binary go? I put it in testbin/ at the workspace root.
• The nu-testbin binary inherits/shares dependencies with the main binary. This isn't a huge issue, but it means that compiling nu-testbin might take longer than necessary. Some Cargo.toml fanegaling might be able to fix this.
• Binaries are only built in certain cases. E.g., cargo test -p <crate> does not build nu or nu-testbin. So, this can cause tests to fail unless cargo test --workspace or cargo b is run beforehand.