t/t2023-checkout-m.sh: fix use of test_must_fail

t/t2023-checkout-m.sh: fix use of test_must_fail

Change an invocation of test_must_fail() to be inside a
test_expect_success() as is our usual pattern. Having it outside
caused our tests to fail under prove(1) since we wouldn't print a
newline before TAP output:

    CONFLICT (content): Merge conflict in both.txt
    # GETTEXT POISON #ok 2 - -m restores 2-way conflicted+resolved file

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