Close stdin where it's not needed in TestLib.pm procedures

Close stdin where it's not needed in TestLib.pm procedures

Where possible, do this using a pseudoterminal, so that things like
openssl that want to open /dev/tty if stdin isn't a tty won't.
Elsewhere, i.e. Windows, just close by providing an empty string using
the standard IPC::Run pipe mechanism.

Patch by Andrew Dunstan, based on an idea from Craig Ringer.

Reviewed by Mark Dilger.

Discussion: https://postgr.es/m/873ebb57-fc98-340d-949d-691b1810bf66@2ndQuadrant.com