tests: make test_expect_code quieter on success

tests: make test_expect_code quieter on success

A command exiting with the expected status is not particularly
notable.

While the indication of progress might be useful when tracking down
where in a test a failure has happened, the same applies to most other
test helpers, which are quiet about success, so this single helper's
output stands out in an unpleasant way.  An alternative method for
showing progress information might to invent a --progress option that
runs tests with "set -x", or until that is available, to run tests
using commands like

	prove -v -j2 --shuffle --exec='sh -x' t2202-add-addremove.sh

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