pgsql: Fix closing stdin in TestLib.pm

Fix closing stdin in TestLib.pm

Commit 9af34f3c6b naively assumed that all non-windows platforms would
have pseudoterminals and that perl would have IO::Pty. Such is not the
case. Instead of assumung anything, test for the presence of IO::Pty and
only use code that might depend on it if it's present. The test result is
exposed in $TestLib::have_io_pty so that it can be conveniently used in
SKIP tests.

Discussion: https://postgr.es/m/20191126044110.GB5435@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/792dba73c8f30528e51c4967d3be8ec4bdc5234b

Modified Files
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src/test/perl/TestLib.pm | 28 +++++++++++++++++++---------
1 file changed, 19 insertions(+), 9 deletions(-)