Move the "check_sub_test_lib_test()" and its sister functions to a new
lib-subtest.sh.
In the future (not in this series) I'd like to test test-lib's output
in a more targeted and smaller test, and I'll need these functions to
do that.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
