t5304: use helper to report failure of "test foo = bar"

t5304: use helper to report failure of "test foo = bar"

For small outputs, we sometimes use:

  test "$(some_cmd)" = "something we expect"

instead of a full test_cmp. The downside of this is that
when it fails, there is no output at all from the script.
Let's introduce a small helper to make tests easier to
debug.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>