pgsql: Fix TestLib::slurp_file() with offset on windows.

Fix TestLib::slurp_file() with offset on windows.

3c5b0685b921 used setFilePointer() to set the position of the filehandle, but
passed the wrong filehandle, always leaving the position at 0. Instead of just
fixing that, remove use of setFilePointer(), we have a perl fd at this point,
so we can just use perl's seek().

Additionally, the perl filehandle wasn't closed, just the windows filehandle.

Reviewed-By: Andrew Dunstan <andrew(at)dunslane(dot)net>
Author: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/20211003173038.64mmhgxctfqn7wl6@alap3.anarazel.de
Backpatch: 9.6-, like 3c5b0685b921

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cd1b2334b8a2c18f54e7ade408c9133805cae557

Modified Files
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src/test/perl/TestLib.pm | 36 ++++++++++++++++++------------------
1 file changed, 18 insertions(+), 18 deletions(-)