Add tests to test__header_value_parser for RFC 2231 parsing code

[f4e67413-755a-47af-885c-5ba33365cf6d]

BPO	17733
Nosy	@bitdancer, @iritkatriel

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = None closed_at = created_at = labels = ['type-feature', 'tests'] title = 'Add tests to test__header_value_parser for RFC 2231 parsing code' updated_at = user = 'https://bugs.python.org/AnkurAnkan' ``` bugs.python.org fields: ```python activity = actor = 'iritkatriel' assignee = 'none' closed = True closed_date = closer = 'iritkatriel' components = ['Tests'] creation = creator = 'Ankur.Ankan' dependencies = [] files = [] hgrepos = [] issue_num = 17733 keywords = [] message_count = 1.0 messages = ['415810'] nosy_count = 3.0 nosy_names = ['r.david.murray', 'Ankur.Ankan', 'iritkatriel'] pr_nums = [] priority = 'normal' resolution = 'out of date' stage = 'resolved' status = 'closed' superseder = None type = 'enhancement' url = 'https://bugs.python.org/issue17733' versions = [] ```

[iritkatriel]

I believe this is about smtpd, which is now deprecated under PEP-594. So I think we can close it.