Add the inventory type to p2p_tx_download context and use it

Summary:
This makes the inventory type and P2P interface part of the context the
tests can send MSG_TX or MSG_AVA_PROOF and count these messages.

There is no change in behavior.

Depends on D9577.

Test Plan:

./test/functional/test_runner.py p2p_tx_download

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9578