[test framework] use an Enum instead of typing.Literal for PortName

Summary:
typing.Literal is new in Python 3.8. We still support Python 3.6 and 3.7.

See D13659#304051

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13671