[avalanche] Explicitely set the legacy format where expected

Summary:
The proof deserialization test uses 2 vectors: one for legacy format and one for the new format. But the test only sets the -legacyavaproof flag for the latter, making the test brittle.

Ref T1676.

Test Plan:

ninja check-avalanche-proof_tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1676

Differential Revision: https://reviews.bitcoinabc.org/D10211