feat(nargo): add support for testing noir libraries

[TomAFrench]

Related issue(s)

Closes #751

Description

Summary of changes

We now only reject compiling libraries if we haven't provided an override for the main function. This allows the test binaries from nargo test to be built while retaining the helpful error message for users running nargo compile on a library.

Dependency additions / changes

N/A

Test additions / changes

N/A

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.
• This PR requires documentation updates when merged.

Additional context

[jfecher]

Good catch. This was an oversight from the original PR