test: test_must_be_empty helper

test: test_must_be_empty helper

There are quite a lot places where an output file is expected to be
empty, and we fail the test when it is not.  The output from running
the test script with -i -v can be helped if we showed the unexpected
contents at that point.

We could of course do

    >expected.empty && test_cmp expected.empty actual

but this is commmon enough to be done with a dedicated helper.

Signed-off-by: Junio C Hamano <gitster@pobox.com>