Fix some issues with TAP tests of pg_basebackup and pg_verify_checksums

Fix some issues with TAP tests of pg_basebackup and pg_verify_checksums

ee9e145 has fixed the tests of pg_basebackup for checksums a first time,
still one seek() call missed the shot.  Also, the data written in files
to emulate corruptions was not actually writing zeros as the quoting
style was incorrect.

Backpatch the portion for pg_basebackup to v11 where these tests have
been introduced.  The tests of pg_verify_checksums are new as of v12.

Author: Michael Banck
Discussion: https://postgr.es/m/1550153276.796.35.camel@credativ.de
Backpatch-through: 11