test_terminal: catch use without TTY prerequisite

test_terminal: catch use without TTY prerequisite

It is easy to forget to declare the TTY prerequisite when
writing tests on a system where it would always be satisfied
(because IO::Pty is installed; see v1.7.3-rc0~33^2, 2010-08-16
for example).  Automatically detect this problem so there is
no need to remember.

	test_terminal: need to declare TTY prerequisite
	test_must_fail: command not found: test_terminal echo hi

test_terminal returns status 127 in this case to simulate
not being available.

Also replace the SIMPLEPAGERTTY prerequisite on one test with
"SIMPLEPAGER,TTY", since (1) the latter is supported now and
(2) the prerequisite detection relies on the TTY prereq being
explicitly declared.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>