Delete outdated "self tests" for the test framework

Summary:
These tests no longer work, and they test trivial stuff that is already
tested in every single existing functional test (starting and stopping
nodes...)

I noticed them when testing mypy, a lint tool that will be introduced when backporting core#18210

See D237

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9271