pgsql: Use native methods to open input in TestLib::slurp_file on Windo

Use native methods to open input in TestLib::slurp_file on Windows.

It is hoped that this will avoid some errors that we have seen before,
but lately with greater frequency, in buildfarm animals.

For now apply only to master. If this proves effective it can be
backpatched.

Discussion: https://postgr.es/m/13900.1572839580@sss.pgh.pa.us

Author: Juan José Santamaría Flecha

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/114541d58e5970e51b78b77b65de16210beaab43

Modified Files
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src/test/perl/TestLib.pm | 27 +++++++++++++++++++++++----
1 file changed, 23 insertions(+), 4 deletions(-)