pgsql: Fix some issues with TAP tests of pg_basebackup

Fix some issues with TAP tests of pg_basebackup

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.

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

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/51be67346ef9dd29dd914ecac220044769efcad6

Modified Files
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src/bin/pg_basebackup/t/010_pg_basebackup.pl | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)