[avalanche] Add height parameter to buildRandomProof

Summary:
As we move to an orphan pool implementation that only accepts proofs with
immature utxos, explicitly specifying proof height will be more common.
This patch makes it easy to do that for random proofs without requiring
the height to be explict in all tests.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11664