Switch pg_verify_checksums back to a blacklist

Switch pg_verify_checksums back to a blacklist

This basically reverts commit d55241af705667d4503638e3f77d3689fd6be31,
leaving around a portion of the regression tests still adapted with
empty relation files, and corrupted cases.  This is also proving to be
failing to check properly relation files located in a non-default
tablespace path.

Per discussion with various folks, including Stephen Frost, David
Steele, Andres Freund, Michael Banck and myself.

Reported-by: Michael Banck
Discussion: https://postgr.es/m/20181021134206.GA14282@paquier.xyz
Backpatch-through: 11