[avalanche] Make send_poll test helper accept optional type argument.

Summary:
Allow the send_poll helper to take an option second argument that is the type of inv to
send a poll for.

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10685