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

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>