[avalanche] Check the utxo limit using the verifyavalancheproof RPC

Summary: This is currently tested via addavalanchenode which is not appropriated. This diffs checks the limit with verifyavalancheproof instead, and the error is already covered with a proper message check.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9794