Merge #11648: [tests] Add messages.py

1135c79 [tests] Tidy up mininode.py module (John Newbery)
f9cd9b1 [tests] Move test_framework Bitcoin primitives into separate module (John Newbery)

Pull request description:

  Second part of #11518.

  Moves the primitive Bitcoin datastructures and message classes into their own module, and tidies up the mininode.py module.

  - First commit is almost entirely move-only
  - Second commit is mostly move-only, but also does a little tidying.

Tree-SHA512: 5d74802677f1ab788e43188653106a96fffd9ab1fe3aa6a4eb94e5807de5dd5c8ee212296f45e8d16c7e3d95cfc4891677e812b7944bd3ab604e04b3b88aa06e

Author: laanwj