Explicitly test parser + normalizer

We use `toml-test` as a catch-all integration test, but we should probably include some explicit tests for various things.

We should also include tests that parser + normalization errors are the errors we expect

Start by reviewing the existing toml-test integration and the parser and normalizer entry points. Identify representative parser and normalization failures with expected errors, then add explicit tests whose passing results demonstrate the requested coverage.

Written by the indexing model from the issue text.