Merge #11707: [tests] Fix sendheaders

Summary:
PR11707 backport https://github.com/bitcoin/bitcoin/pull/11707/files
9d42cc333 [tests] address review comments (John Newbery)
f0c4ab9a7 [tests] fix flakiness in sendheaders.py (John Newbery)
25fd6e2c2 [tests] refactor check_last_announcement() in sendheaders.py (John Newbery)
f39d4bbd1 [tests] tidy up BaseNode in sendheaders.py (John Newbery)
2613c545f [tests] fix flake8 warnings in sendheaders.py (John Newbery)

Pull request description:

This PR should fix the intermittent failure of sendheaders.py described in #11673. The first three commits are tidying up and refactoring the file. The final commit _fix flakiness in sendheaders.py_ fixes the intermittent failures. The commit message for that commit describes the problems that are being fixed.

I think @laanwj @MeshCollider @MarcoFalke have seen these failures.

fixes #11673

Backport note:

• Many of these were just codestyle changes so the Diff here looks much smaller than the PR

Test Plan: test_runner.py

Reviewers: jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T452

Differential Revision: https://reviews.bitcoinabc.org/D3085