Add basic TAP tests for psql's tab-completion logic.

Up to now, psql's tab-complete.c has had exactly no regression test
coverage.  This patch is an experimental attempt to add some.

This needs Perl's IO::Pty module, which isn't installed everywhere,
so the test script just skips all tests if that's not present.
There may be other portability gotchas too, so I await buildfarm
results with interest.

So far this just covers a few very basic keyword-completion and
query-driven-completion scenarios, which should be enough to let us
get a feel for whether this is practical at all from a portability
standpoint.  If it is, there's lots more that can be done.

Discussion: https://postgr.es/m/10967.1577562752@sss.pgh.pa.us

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