Rename p2p_tx_download to p2p_inv_download

Summary:
This is the last step toward making the test more generic so we can
reuse it for testing the proof relaying. The timing file is manually
updated to prevent the test from slipping into the regular test suite.

Depends on D9579.

Test Plan:

./test/functional/test_runner.py p2p_inv_download

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9580