[avalanche] Make sure the master key matches the proof master

Summary:
As per title. This would cause the addLevel() to fail but only if the
session key mismatch the proof master. Adding an explicit test for this
case will always catch the issue. This will be a better solution than
D9668 when we will start to supply delegations as well.

Ref T1635.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1635

Differential Revision: https://reviews.bitcoinabc.org/D9669