t6300: use test_atom for some un-modern tests

t6300: use test_atom for some un-modern tests

Because this script has to test so many formatters, we have
the nice "test_atom" helper, but we don't use it
consistently. Let's do so. This is shorter, gets rid of some
tests that have their "expected" setup outside of a
test_expect_success block, and lets us organize the changes
better (e.g., putting "refname:short" near "refname").

We also expand the "%(push)" tests a little to match the
"%(upstream)" ones.

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