Merge #11707: [tests] Fix sendheaders

9d42cc3 [tests] address review comments (John Newbery)
f0c4ab9 [tests] fix flakiness in sendheaders.py (John Newbery)
25fd6e2 [tests] refactor check_last_announcement() in sendheaders.py (John Newbery)
f39d4bb [tests] tidy up BaseNode in sendheaders.py (John Newbery)
2613c54 [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

Tree-SHA512: 278e1af85f2eae00f970f2d8ef33686dd52b4f62180dea4cfdaff7bcf3287c6f1c2930355d99461a12f0c51c4d42cc3b1cb3275174134028ca4d06ffc24c18dd

Author: MarcoFalke