checkout tests: don't ignore "git <cmd>" exit code

checkout tests: don't ignore "git <cmd>" exit code

Change a fragile pattern introduced in 696acf45f96 (checkout:
implement "-" abbreviation, add docs and tests, 2009-01-17) to check
the exit code of both "git symbolic-ref" and "git rev-parse".

Without this change this test will become flaky e.g. under
SANITIZE=leak if some (but not all) memory leaks revealed by these
commands are fixed.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>