Merge #14244: amount: Move CAmount CENT to unit test header

Summary:
fa84723e73 amount: Move CAmount CENT to unit test header (MarcoFalke)

Pull request description:

`CAmount` is currently not type-safe. Exporting a constant (`CENT`) that is commonly not referred to by that name might be confusing. `CENT` is only used in two places prior to this commit (`ParseMoney` and `MIN_CHANGE`). So replace these with constants relative to `COIN` and move `CENT` to the unit test header.

Tree-SHA512: 5273e96d8664ced6ae211abde2e20bc763e6e99f89404eec02c621f29e1d235e5f9b1ade933743843fae16fc24b643f883deda9221e3d9fd31229d2ab63a914f

Backport of Core PR14244

Test Plan:

ninja check
ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5489