Merge #13522: [tests] Fix p2p_sendheaders race

Summary:
PR13522 backport https://github.com/bitcoin/bitcoin/pull/13522/files
75848bcf40 [tests] Fix p2p_sendheaders race (John Newbery)

Pull request description:

p2p_sendheaders has a race in part 1.3.

part 1.2 sends a block to the node over the 'test_node' connection, but
doesn't wait for an inv to be received on the 'inv_node' connection. If
we get to part 1.3 before that inv has been received, then the
subsequent call to check_last_inv_announcement could fail.

Depends on D3088

Test Plan: test_runner.py

Reviewers: jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T454

Differential Revision: https://reviews.bitcoinabc.org/D3089