feat(prepro): Add metadata processing tests

[anna-parker]

resolves #

preview URL: https://test-prepro-v2.loculus.org/

Summary

This PR:

• renames process_date to parse_and_assert_past_date to make function clearer
• adds prepro metadata processing tests to the CI
• changes function checks from if date is None: to if not date to handle case when input is ""
• Makes error/warning messages clearer by always adding the output_field
• Removes outdated tests

It also uses @fengelniederhammer's suggestion for structuring preprocessing tests: #2888 (review).

• adds factory methods for generating UnprocessedEntry and ProcessedEntry objects
• Creates a test structure, with UnprocessedEntry as input and ProcessedEntry as expected_output and confirms the output of process_all corresponds to expected_output.

Screenshot

PR Checklist

• All necessary documentation has been adapted.
• The implemented feature is covered by an appropriate test.