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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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9e3be5c182d71d7575aa2e63d9604bfa715a432e

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