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