Several off-by-more-than-one errors caused the output in case of a
test failure to be truncated and unintelligible.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/6a7a8516-7d11-8fbd-0e8b-eadb4f0679eb%402ndquadrant.com