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

src/test/perl/PostgresNode.pm

@@ -1534,6 +1534,73 @@ sub psql
 
 =pod
 
+=item $node->interactive_psql($dbname, \$stdin, \$stdout, $timer, %params) => harness
+
+Invoke B<psql> on B<$dbname> and return an IPC::Run harness object,
+which the caller may use to send interactive input to B<psql>.
+The process's stdin is sourced from the $stdin scalar reference,
+and its stdout and stderr go to the $stdout scalar reference.
+ptys are used so that psql thinks it's being called interactively.
+
+The specified timer object is attached to the harness, as well.
+It's caller's responsibility to select the timeout length, and to
+restart the timer after each command if the timeout is per-command.
+
+psql is invoked in tuples-only unaligned mode with reading of B<.psqlrc>
+disabled.  That may be overridden by passing extra psql parameters.
+
+Dies on failure to invoke psql, or if psql fails to connect.
+Errors occurring later are the caller's problem.
+
+Be sure to "finish" the harness when done with it.
+
+The only extra parameter currently accepted is
+
+=over
+
+=item extra_params => ['--single-transaction']
+
+If given, it must be an array reference containing additional parameters to B<psql>.
+
+=back
+
+This requires IO::Pty in addition to IPC::Run.
+
+=cut
+
+sub interactive_psql
+{
+	my ($self, $dbname, $stdin, $stdout, $timer, %params) = @_;
+
+	my @psql_params = ('psql', '-XAt', '-d', $self->connstr($dbname));
+
+	push @psql_params, @{ $params{extra_params} }
+	  if defined $params{extra_params};
+
+	# Ensure there is no data waiting to be sent:
+	$$stdin = "" if ref($stdin);
+	# IPC::Run would otherwise append to existing contents:
+	$$stdout = "" if ref($stdout);
+
+	my $harness = IPC::Run::start \@psql_params,
+	  '<pty<', $stdin, '>pty>', $stdout, $timer;
+
+	# Pump until we see psql's help banner.  This ensures that callers
+	# won't write anything to the pty before it's ready, avoiding an
+	# implementation issue in IPC::Run.  Also, it means that psql
+	# connection failures are caught here, relieving callers of
+	# the need to handle those.  (Right now, we have no particularly
+	# good handling for errors anyway, but that might be added later.)
+	pump $harness
+	  until $$stdout =~ /Type "help" for help/ || $timer->is_expired;
+
+	die "psql startup timed out" if $timer->is_expired;
+
+	return $harness;
+}
+
+=pod
+
 =item $node->poll_query_until($dbname, $query [, $expected ])
 
 Run B<$query> repeatedly, until it returns the B<$expected> result