Improve coverage of SHA256 SelfTest code
Improve coverage of SHA256 SelfTest code

Summary:
This is a backport of Core's PR 13438

Depends on D1846

Test Plan:
  make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1847

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