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

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

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>
---
 t/t2023-checkout-m.sh |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/t/t2023-checkout-m.sh b/t/t2023-checkout-m.sh
index 1a40ce0..7e18985 100755
--- a/t/t2023-checkout-m.sh
+++ b/t/t2023-checkout-m.sh
@@ -17,7 +17,9 @@ test_expect_success setup '
 	test_commit added_in_topic each.txt in_topic
 '
 
-test_must_fail git merge master
+test_expect_success 'git merge master' '
+    test_must_fail git merge master
+'
 
 clean_branchnames () {
 	# Remove branch names after conflict lines
-- 
1.7.7.3