Pick better names for the tests in p2p_tx_download.py

Summary:
Now that there is a test context that can be used with transactions or
proof, rename the variables, functions and comments to reflect this. The
last step is to rename the file. There is no change in behavior.

Depends on D9578.

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/D9579