describe tests: don't rely on err.actual from "check_describe"

Convert the one test that relied on the "err.actual" file produced by check_describe() to instead do its own check of "git describe" output. This means that the two tests won't have an inter-dependency (e.g. if the earlier test is skipped). An earlier version of this patch instead asserted that no other test had any output on stderr. We're not doing that here out of fear that "gc --auto" or another future change to "git describe" will cause it to legitimately emit output on stderr unexpectedly[1]. I'd think that inverting the test added in 3291fe4072e (Add git-describe test for "verify annotated tag names on output", 2008-03-03) to make checking that we don't have warnings the rule rather than the exception would be the sort of thing the describe tests should be catching, but for now let's leave it as it is. 1. http://lore.kernel.org/git/xmqqwnuqo8ze.fsf@gitster.c.googlers.com Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>