[avalanche] Improve proof verification test coverage

Summary:
The utxo-missing-or-spent case was missing from the test.

Ref T1611.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1611

Differential Revision: https://reviews.bitcoinabc.org/D9658