test_terminal: ensure redirections work reliably

For terminal tests that capture output/stderr, the TTY prerequisite
warning does not quite work for commands like

	test_terminal foo >out 2>err

because the warning gets "swallowed" up by the redirection that's
supposed only to be done by the subcommand.

Even worse, the outcome depends on whether stdout was already a
terminal (in which case test_terminal is a noop) or not (in which case
test_terminal introduces a pseudo-tty in the middle of the pipeline).

	$ test_terminal.perl sh -c 'test -t 1 && echo >&2 YES' >out
	YES
	$ sh -c 'test -t 1 && echo >&2 YES' >out
	$

So:

 - use the test_terminal script even when running with "-v".

 - skip tests that require a terminal when the test_terminal
   script is unusable because IO::Pty is not installed.

 - write the "need to declare TTY prerequisite" message to fd 4,
   where it will be printed when running tests with -v, rather
   than being swallowed up by an unrelated redireciton.

Noticed-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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