Coverage: missing tests (protocol)

[anarthal]

• compute_column_type json (MariaDB specific?)
• deserialize_execute_response marks some error codes as not hit
• deserialize_text_ymd: separator
• deserialize_text_value_date: excess characters
• deserialize_text_value_datetime: separator
• deserialize_text_value_time: separator
• deserialize_text_value_float: res.ptr != end
• deserialize_binary_field_string: error
• should_perform_full_auth: branch
• next_char_utf8mb4: not enough size checks

See if we could write some of these in a more coverage-friendly way.