[avalanche] Add some test coverage for the buildavalancheproof RPC

Summary:
The RPC is used everywhere in the tests but is not tested itself.
This diff does not intend to extend the safety checks but only check for the existing errors to prevent regressions.

Test Plan:

./test/functional/test_runner.py abc_rpc_buildavalancheproof

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D10084