describe tests: improve test for --work-tree & --dirty

describe tests: improve test for --work-tree & --dirty

Improve tests added in 9f67d2e8279 (Teach "git describe" --dirty
option, 2009-10-21) and 2ed5c8e174d (describe: setup working tree for
--dirty, 2019-02-03) so that they make sense in combination with each
other.

The "check_describe" being removed here was the earlier test, we then
later added these --work-tree tests which really just wanted to check
if we got the exact same output from "describe", but the test wasn't
structured to test for that.

Let's change it to do that, which both improves test coverage and
makes it more obvious what's going on here.

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