Add a test for trying to run `cargo open` without a `Cargo.toml`

[carols10cents]

In main.rs, add a unit test for the cargo_dir function that checks that it panics if we try to run this command in a directory that doesn't have a Cargo.toml and thus would have no idea where to look to figure out where that crate might be!

Use the existing test marked should_panic for an example of how to make a test check that a function panics.

I doubt that this test will pass currently! Eventually, I'd like this to panic with a message like "Cannot find Cargo.toml file in the current directory" or similar.