t/test-lib.sh: add test_external and test_external_without_stderr

t/test-lib.sh: add test_external and test_external_without_stderr

This is for running external test scripts in other programming
languages that provide continuous output about their tests.  Using
test_expect_success (like "test_expect_success 'description' 'perl
test-script.pl'") doesn't suffice here because test_expect_success
eats stdout in non-verbose mode, which is not fixable without major
file descriptor trickery.

Signed-off-by: Lea Wiemann <LeWiemann@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>