t0000: replace test_must_fail with run_sub_test_lib_test_err()

t0000: replace test_must_fail with run_sub_test_lib_test_err()

The test_must_fail function should only be used for git commands since
we should assume that external commands work sanely. We use
test_must_fail to test run_sub_test_lib_test() but that function does
not invoke any git commands internally. Even better, we have a function
that's exactly meant to be used when we expect to have a failing test
suite: run_sub_test_lib_test_err()!

Replace `test_must_fail run_sub_test_lib_test` with
`run_sub_test_lib_test_err`.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>