t/t5319-multi-pack-index.sh: drop early 'test_done'

t/t5319-multi-pack-index.sh: drop early 'test_done'

In 6ce9d558ced (midx-write: skip rewriting MIDX with `--stdin-packs`
unless needed, 2025-12-10), an extra 'test_done' was added, causing the
test script to finish before having run all of its tests.

Dropping this extraneous 'test_done' exposes a bug from commit
6ce9d558ced that causes a subsequent test to fail. Mark that test with a
'test_expect_failure' for now, and the subsequent commit will explain
and fix the bug.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>