Add TAP tests for pg_verify_checksums

Add TAP tests for pg_verify_checksums

All options available in the utility get coverage:
- Tests with disabled page checksums.
- Tests with enabled test checksums.
- Emulation of corruption and broken checksums with a full scan and
single relfilenode scan.

This patch has been contributed mainly by Michael Banck and Magnus
Hagander with things presented on various threads, and I have gathered
all the contents into a single patch.

Author: Michael Banck, Magnus Hagander, Michael Paquier
Reviewed-by: Peter Eisentraut
Discussion: https://postgr.es/m/20181005012645.GE1629@paquier.xyz