tests: make use of the test_must_be_empty function

Change various tests that use an idiom of the form:

    >expect &&
    test_cmp expect actual

To instead use:

    test_must_be_empty actual

The test_must_be_empty() wrapper was introduced in ca8d148daf ("test:
test_must_be_empty helper", 2013-06-09). Many of these tests have been
added after that time. This was mostly found with, and manually pruned
from:

    git grep '^\s+>.*expect.* &&$' t

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

t/t2024-checkout-dwim.sh

@@ -24,9 +24,8 @@ test_branch_upstream () {
 }
 
 status_uno_is_clean () {
-	>status.expect &&
 	git status -uno --porcelain >status.actual &&
-	test_cmp status.expect status.actual
+	test_must_be_empty status.actual
 }
 
 test_expect_success 'setup' '

t/t2025-worktree-add.sh

@@ -412,9 +412,8 @@ test_expect_success '"add" <path> <branch> dwims with checkout.defaultRemote' '
 		git fetch repo_upstream2 &&
 		test_must_fail git worktree add ../foo foo &&
 		git -c checkout.defaultRemote=repo_upstream worktree add ../foo foo &&
-		>status.expect &&
 		git status -uno --porcelain >status.actual &&
-		test_cmp status.expect status.actual
+		test_must_be_empty status.actual
 	) &&
 	(
 		cd foo &&