tests: fix diff order arguments in test_cmp

tests: fix diff order arguments in test_cmp

Fix the argument order for test_cmp. When given the expected
result first the diff shows the actual output with '+' and the
expectation with '-', which is the convention for our tests.

Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>