test-lib tests: don't provide a description for the sub-tests

test-lib tests: don't provide a description for the sub-tests

Change the $test_description provided for the generated subtests to be
constant, since the only purpose of having it is that test-lib.sh will
barf if it isn't supplied.

The other purpose of having it was to effectively split up the test
description between the argument to test_expect_success and the
argument to "write_and_run_sub_test_lib_test". Let's only use one of
the two.

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