test: Seed test RNG context for each test case, print seed

Summary:
Debugging failing unit tests is hard if the failure is non-deterministic and the seed is not known.

Fix that by printing the seed and making it possible to set the seed from outside.

Backport of Core PR16978

Test Plan: ninja && ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7970