[avalanche] Bump stake used in proof deserialization tests

Summary:
Bumping the dust threshold is likely to break these tests even though the stakes currently exceed the dust threshold by a reasonable margin. I've bumped them by another order of magnitude since I expect the new dust threshold to exceed these current values.

While preemptive, reviewing this piece separately helps simplify the threshold bump patch and can be reviewed in tandem with other necessary components like D11750 without rebasing diffs.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11821