[avalanche] Sync proofs when functional tests call sync_all

Summary:
Now that avalanche is enabled by default the test framework can treat it as
such. It is natural to expect sync_all to sync all inventories, including
proofs.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12999