fix: correct test for mutually exclusive feature flags

[TomAFrench]

Related issue(s)

Resolves #1084

Description

Summary of changes

I've updated the checks to match the feature flags which are in use.

I've also removed the default feature flag for the nargo package. This should be set in the nargo_cli crate (and by any direct consumers of nargo)

Dependency additions / changes

Test additions / changes

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Documentation needs

• This PR requires documentation updates when merged.

Additional context

[jfecher]

👍

[phated]

Why did you remove the default?

[TomAFrench]

@phated and I came to the conclusion that we should be defining the backends in the nargo_cli crate and then passing this through into nargo. This is necessary to avoid conflicting backend feature flags when we're using nargo as a dependency of noir-wasm (as this would rely on the wasm backend)