[tests] Tidy up mininode

Summary:
Add docstrings and renames some methods.
Also removes the redundant NodeConn.readable() method override.

Partial backport of core PR11712 (commit 4d50598)

Depend on D2357

Test Plan:

./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2358