Add a context for running the p2p_tx_download test

Summary:
Since transactions and proof relaying will have very close features,
most of the tests can be used for testing boths. This diff creates a
context object which will contain all the tx/proof only elements so the
tests can be used against both kind of inventory while keeping the logic
unchanged. As a first step, the networking constants are added to the
context. More will be added later until completion.

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/D9577