[avalanche] Split the poll_and_response test in two

Summary:
This diff extracts the test for an invalid block not being polled out of the more generic test. Similar to D10260, this is a preliminary stop before converting the poll_and_response to use the context.

Ref T1854.

Test Plan:

ninja check-avalanche-processor_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10268