Avoid inflight polls in getavaaddr_interval_test

Summary: Inflight polls during the availability score retrieval can cause the value to change and the test to fail reaching the expected condition. Fix this by exhausing the polls before checking the score (which might be zero but not negative because our peers always answer the polls).

Test Plan:

./test/functional/test_runner.py abc_p2p_getavaaddr

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17698