Fix max_early_data_size handling

Looking at the coverage report, I realized that gnutls_record_{get,set}_max_early_data_size is not tested. This adds a test and fixes the uncovered issues:

• the client ignored max_early_data_size advertised in NST
• gnutls_record_send_early_data to respect the advertised maximum and return GNUTLS_E_RECORD_LIMIT_REACHED when it exceeds

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code