Test error spans in your integration tests

[juntyr]

I wonder if we could also add similar span tests to the following tests:

• https://github.com/ron-rs/ron/blob/master/tests/256_comma_error.rs
• https://github.com/ron-rs/ron/blob/master/tests/393_serde_errors.rs
• https://github.com/ron-rs/ron/blob/master/tests/203_error_positions.rs

Originally posted by @juntyr in https://github.com/ron-rs/ron/issues/569#issuecomment-2929698861