TAP tests for pg_verify_checksums

Hi all,

The topic of $subject has been discussed a bit times, resulting in a
couple of patches on the way:
https://www.postgresql.org/message-id/20180830200258.GG15446@paquier.xyz
https://www.postgresql.org/message-id/CABUevEzEKrwPeGK2o-OV4z2MjfT6Cu8cLfA-v1S1j4z8x7WJjg@mail.gmail.com

However nothing has actually happened.  Based on the previous feedback,
attached is an updated patch to do the actual job.

Magnus Hagander and Michael Banck need to be credited as authors, as
this patch is roughly a merge of what they proposed.

Thoughts?
--
Michael