[avalanche] Consistently use the FromHex methods in tests

Summary:
This has a few benefits:

• It removes the dependency to streams.h
• It prevents the tests running into a meaningless exception in the input stream is invalid
• It makes it easier to update to new format in the future

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11329