More nuon tests, fix table print

[sophiajt]

Description

This PR does two separate things:

• Adds a sample.nuon file to our test formats and adds some tests to check it.
• Fixes table printing of mixed lists, so we don't treat a list with a single record as somehow making everything a table

Tests

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --all --all-features -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo build; cargo test --all --all-features to check that all the tests pass