[move-only] Move constants to test-only header

Summary:
Partial backport of core#22950:
https://github.com/bitcoin/bitcoin/pull/22950/commits/40acd6fc9a8098fed85abf4fb727a5f0dff8a2ff

Depends on D12332.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12333