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_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c4465cd09e3ab034ced25e623d5760e9ce437f6c

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