Merge #13467: [Tests] Make p2p_segwit easier to debug

e3aab29 [tests] p2p_segwit: sync_blocks in subtest wrapper. (John Newbery)
55e8050 [tests] p2p_segwit: remove unnecessary arguments from subtests. (John Newbery)
25711c2 [tests] p2p_segwit: log and assert segwit status in subtest wrapper. (John Newbery)
6839863 [tests] p2p_segwit: Make sure each subtest leaves utxos for the next. (John Newbery)
bfe3273 [tests] p2p_segwit: wrap subtests with subtest wrapper. (John Newbery)
2af4e39 [tests] p2p_segwit: re-order function definitions. (John Newbery)
94a0134 [tests] p2p_segwit: standardise comments/docstrings. (John Newbery)
f7c7f8e [tests] p2p_segwit: Fix flake8 warnings. (John Newbery)

Pull request description:

  `p2p_segwit.py` is a very long test, composed of multiple subtests. When it fails it's difficult to debug for a couple of reasons:

  - Control flow jumps between different methods in the test class, so it's a little difficult to follow the code.
  - state may be carried forward unintentionally from one subtest to the next.

  Improve that by wrapping the subtests with a `@subtest` decorator which:
  - logs progress
  - asserts state after each subtest

  As usual, I've also included a few commits which generally tidy up the test and improve style.

Tree-SHA512: 3650602b3ce9823dc968cc5f2e716757feadc3dbedb3605eb79bb3df91a6db8ae53431f253b440da690e3a8e9d76de84fad4368a2663aeb40e6b9427cf948870

Author: laanwj