test-lib: make test_expect_code a test command

Change test_expect_code to be a normal test command instead of a
top-level command.

As a top-level command it would fail in cases like:

test_expect_code 1 'phoney' '
foo && bar && (exit 1)
'

Here the test might incorrectly succeed if "foo" or "bar" happened to
fail with exit status 1. Instead we now do:

test_expect_success 'phoney' '
foo && bar && test_expect_code 1 "(exit 1)"
'

Which will only succeed if "foo" and "bar" return status 0, and "(exit
1)" returns status 1.  Note that test_expect_code has been made slightly
noisier, as it reports the exit code it receives even upon success.

Some test code in t0000-basic.sh relied on the old semantics of
test_expect_code to test the test_when_finished command. I've
converted that code to use an external test similar to the TODO test I
added in v1.7.3-rc0~2^2~3.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>