tests: make use of the test_must_be_empty function

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>