Split off key_io_tests from base58_tests

Summary:
This is a partial backport of Core PR11372 : https://github.com/bitcoin/bitcoin/pull/11372/commits/92f1f8b3197c2ba3cf65556070509838098975a4

Depends on D3577

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3578