Merge #14543: [QA] minor p2p_sendheaders fix of height in coinbase

Summary:
1cdb9bb51f minor p2p_sendheaders fix of height in coinbase (Gregory Sanders)

Pull request description:

> \# Now announce a header that forks the last two blocks

Doesn't effect any behavior since BIP34 isn't active in regtest for many blocks.

Tree-SHA512: 3f214b956a94250bb640f63b6ff707930e1d4cb8df1bbf0fef4012d89a94bafbde0d7b42bbe7113ec33810169281c22c6e389445921d99decb74aa56e87a0f27

Backport of Core PR14543

Test Plan: test_runner.py p2p_sendheaders

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6774