tests: factor out terminal handling from t7006

Other tests besides the pager ones may want to check how we handle
output to a terminal. This patch makes the code reusable.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-terminal.sh

@@ -0,0 +1,28 @@
+#!/bin/sh
+
+test_expect_success 'set up terminal for tests' '
+	if test -t 1
+	then
+		>stdout_is_tty
+	elif
+		test_have_prereq PERL &&
+		"$PERL_PATH" "$TEST_DIRECTORY"/test-terminal.perl \
+			sh -c "test -t 1"
+	then
+		>test_terminal_works
+	fi
+'
+
+if test -e stdout_is_tty
+then
+	test_terminal() { "$@"; }
+	test_set_prereq TTY
+elif test -e test_terminal_works
+then
+	test_terminal() {
+		"$PERL_PATH" "$TEST_DIRECTORY"/test-terminal.perl "$@"
+	}
+	test_set_prereq TTY
+else
+	say "# no usable terminal, so skipping some tests"
+fi