Merge #14571: [tests] Test that nodes respond to getdata with notfound

Summary:
PR14571 backport https://github.com/bitcoin/bitcoin/pull/14571/files
fa78a2fc67 [tests] Test that nodes respond to getdata with notfound (MarcoFalke)

Pull request description:

If a node has not announced a tx at all, then it should respond to
getdata messages for that tx with notfound, to avoid leaking tx
origination privacy.

In the future this could be adjusted such that a node responds with
notfound when a tx has not been announced to us, but that seems
to be a more involved change. See e.g.
https://github.com/jnewbery/bitcoin/commits/pr14220.1

Backport note: this was backported a bit early, but it runs nicely with
two extra lines included (as marked in comment).

Test Plan: test_runner.py

Reviewers: jasonbcox, Fabien, deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3614