test: use a dataclass in p2p_inv_download

Summary:
This removes some boilerplate code, and we can emulate immutability with frozen=True.

Also, define the default values in the class body for the constants that are identical for all 3 contexts. And use the cosntants defined in p2p.py when available for these defaut values.

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18652