[avalanche] Fix flaky checks after orphaning a valid proof

Summary: This test occasionally fails when UpdatedBlockTip has been queued but not yet called, resulting in the proof still returning as not orphaned during the getrawavalancheproof call.

Test Plan:

for i in {1..20} ; do ./test/functional/test_runner.py abc_rpc_avalancheproof.py ; done

Reviewers: #bitcoin_abc, PiRK, Fabien

Reviewed By: #bitcoin_abc, PiRK, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11734